Just for the sake of comparison, I thought it would be interesting to review the practice in some other kingdoms about titles and styles of husbands of royal princesses or reigning queens, as it could provide some contrast to what is done in the UK and what might be done in Jack's case specifically. If it is too much off-topic, please feel free to move this post to another forum or delete it.
1) Belgium: prior to 1991, husbands of Princesses of Belgium didn't automatically get any royal title or style. By contrast, wives of Princes of Belgium were automatically Princesses of Belgium (in their own right, referred to by their own name) and HRHs. After 1991, however, neither husbands nor wives of royal princes or princesses are automatically titled, but they can still receive a title by a separate royal decree that applies only to them specifically. The husband of Princess Astrid already held foreign titles of his own as a member of the House of Austria-Este (although those titles are not legally recognized in their countries of origin anymore); nonetheless, King Albert II made him a Prince of Belgium and an HRH in his own right, giving him the same treatment that was given to his daughters-in-law (Mathilde and Claire). The wife of Prince Amedeo, the first grandson of King Albert II to get married, did not get a royal title,but the Royal Court refers to her by courtesy as
Princess Amedeo. Traditionally, the wife of the eldest son of the King was called Duchess of Brabant in addition to Princess of Belgium (as was the case with Mathilde); now that the eldest daughter of the King has become Duchess of Brabant
suo jure , it is unclear how her husband will be called , even though it is all but sure that he will be at least a Prince of Belgium and an HRH. The wife of the King of the Belgians, on the other hand, always had the courtesy title of Queen and the style of Majesty; now that women can ascend the throne, it is unclear how the husband of the Queen of the Belgians would be called, but I assume he would probably have the dignity of a prince only.
2) The Netherlands: since 2002, the law on membership of the Royal House explicitly provides for the husband of the reigning queen and the husband of the heiress to the throne (now the Princess of Orange) to be created Princes of the Netherlands by royal decree, which normally carries a style of HRH with it. Husbands and wives of other Princes or Princesses of the Netherlands (explicitly in the law, children of the monarch or children of the heir to the throne) are not
explicitly included in the category of people who can be made Princes or Princesses of the Netherlands in their own right by separate royal decree, but as long as their spouses are members of the Royal House, there is actually a loophole in the law under which they could still be theoretically possible, albeit unlikely. Historically, wives of Princes of the Netherlands who are not princesses in their own right like Laurentien for example have been, however, referred to by courtesy as Princesses of the Netherlands and HRHs, and by their own name unlike in the UK; that is not the case, however, for husbands of Princesses of the Netherlands
suo jure who are not titled (the husband of Princess Margriet, for example, has neither a legal title nor is referred to by a courtesy title). The situation of grandchildren of the monarch who are not children of the heir is trickier; under the 2002 law, they are not normally expected to be created Princes/Princesses of the Netherlands or HRHs by separate royal decree, although, again, that is still technically possible as long as they are members of the Royal House, as is also still technically possible to make them Princes/Princesses of Orange-Nassau on a personal capacity, which normally carries with it the style of HH. The practice under Queen Beatrix was, however, to give them instead a title of nobility in the rank of count or countess, hereditary in male line only. In that case, the normal naming rules in the Dutch nobility would apply to their spouses, i.e. the wife of a count uses her husband's title, but the husband of a countess does not use his wife's title.
3) Spain: the 1987 royal decree on titles and styles of the royal family actually lays down clear rules about how royal husbands should be called. The husband of the reigning queen is now only a prince with the style of HRH (previously, husbands of reigning queens in Spain were called King with the style of His Majesty). The husband of the Princess of Asturias, however, has the right to use all titles and styles of his wife meaning that, at least under current regulations (which can be changed in the future), he would not only be an HRH, but also Prince of Asturias (and of Gerona, Viana, etc.). On the other hand, under the royal decree, neither husbands of infantas nor wives of infantes get any title or style automatically. In the Spanish peerage, where women, unlike in the UK, can and do inherit titles, it is customary for husbands of peeresses to use the titles of their wives; the ducal titles held by infantes and infantas are legally distinct, however, because they are not hereditary peerages, but rather "titles of nobility belonging to the Crown" which, according to the 1987 royal decree, can be used by members of the Royal Family only, by grace of the monarch, and strictly on a personal basis. So, technically, the legal experts say that it is
wrong to refer to the husband of an infanta by the ducal title of his wife; nonetheless, probably by analogy with the standard practice in the peerage, the husbands of both Infanta Elena and Infanta Cristina were routinely called Duke of Lugo and Duke of Palma de Mallorca in the Spanish press. I assume it won't be different with Infanta Sofía's husband. Children of infantes or infantas, on the other hand, including all of JC's grandchildren other than Felipe's daughters, are not HRHs under the 1987 decree, but rather only Grandees of Spain with the style His/Her Excellency; as I understand it, the husbands of JC's granddaughters, other than obviously the husbands of the Princess of Asturias and the Infanta Sofía, will be untitled either by law or by courtesy.
4) Sweden: always a tricky case due to the lack of clear legal rules. Before 1980, the wife of the King was "Sweden's Queen" (
Sveriges Drottning), which is still the case for Queen Silvia. Now that "Sweden's Queen" is a title that can be also used by the person who occupies the throne, it is unclear how her husband would be called, but I assume he would be just a prince and an HRH. As far as princes and princesses are concerned, again prior to 1980, the heir to throne was the Crown Prince and his wife was called Crown Princess; a male descendant of King Karl XIV Johan in male line other than the Crown Prince was called a "Hereditary Prince" (
Arvfurste) as long as he was not excluded from the succession; both a wife of an Arvfurste and a female descendant in male line of the first Bernadotte king had the rank of princess (
Prinsessa, not
Arvfurstinna), but the husbands of princesses of the blood got no royal titles or styles. Under the new post-1980 system, the husband of the current titular Crown Princess is called "Prince of Sweden" (
Prins av Sverige) with the style of HRH and also uses her wife's personal ducal title (being referred to as Duke of Västergötland then), but is
not called "Crown Prince"; it is unclear how the wife of a male titular Crown Prince would be called. The wives of other Princes of Sweden by birth (I mean, other than the Crown Prince), based on Sofia's precedent, are apparently to be called Princesses of Sweden with the HRH style and the right to use their husband's ducal title. Reportedly, the same treatment was offered to Chris O'Neill when he married Princess Madeleine, but he declined it because he was allegedly told by the Court that, in order to become a Prince of Sweden, he had to take up Swedish citizenship, which he didn't want to do. Please note that the King of Sweden lost the power to confer titles of nobility in 1975, so both the titles of Prince/Princess of Sweden and Duke/Duchess of [one or more Swedish provinces] are titles of the Royal House, rather than titles of nobility. Apparently, as we have found out recently in these forums, those titles are not even mentioned in Swedish documents like birth certificates, although the designation of "prince or princess of the Royal House" is used in the Swedish Act of Succession (which is part of the constitution) to refer to people in line to the throne and, therefore, I would assume is a legally recognized title.
Please feel free to add information about Denmark, Luxembourg and Norway. I am not particularly knowledgeable about those realms, especially historically.
The rule in the UK is that grandchildren of a sovereign in male line are princes/princesses and HRHs, but grandchildren in maternal line like Peter or Zara don't get any royal title. As for spouses, the general rule in the UK is that wives automatically use the titles of their husbands by courtesy, but husbands don't use the titles of their wives (that is true both in the peerage and in the Royal Family).
PS: Just for the sake of those who might not know, infantes or infantas of Spain are, ordinarily under the 1987 decree, the children of the monarch other than the Prince (or Princess ) of Asturias, and the children of the Prince (or Princess) of Asturias. On the other hand, since December 2015, only persons who are born as children or grandchildren of the King, or as children or grandchildren of the heir to the throne become automatically new Princes or Princesses of Belgium.