Loose indeed! Well, if all the adults who are on the balcony each year for the Trooping turn up...
My understanding of "all the royal family" is:
- The Queen and the Duke of Edinburgh.
- The Prince of Wales and the Duchess of Cornwall.
- The Duke and Duchess of Cambridge.
- The Duke of York.
- The Earl of Wessex and the Countess of Wessex.
- The Princess Royal.
- The Duke and Duchess of Gloucester.
- The Duke and Duchess of Kent.
- Princess Alexandra.
- And, perhaps, Prince and Princess Michael of Kent.
I don't expect Prince Henry of Wales, or Princesses Beatrice or Eugenie of York to be present.
This state visit is important in many levels. First, Spain is now the 4th largest economy in the EU-27 after Germany, France and Italy, a key EU member, and a major foreign investor in the UK. Second, Spain is one of the EU countries with the largest number of resident British expatriates, whose fate is now uncertain under Brexit. Third, Gibraltar will possibly be a sticking point with the EU during the Brexit negotiations.
Finally, politics aside, after the fall of Russian, French, German , Portuguese and Austrian monarchies, Britain and Spain are the two only remaining ancient European monarchies with an imperial past and, therefore, a state visit from the sovereign of one of those countries to the other is pretty much the mother of all state visits as far as European monarchies are concerned. Just to clarify my statement, both Belgium and the Netherlands also had significant colonial empires of their own, but they are technically "new" monarchies that were only established in the 19th century; Sweden and Denmark on the other hand are old monarchies, but none of them had an imperial past comparable to that of Britain and Spain (or even the Netherlands).
The significance of the visit is highlighted by the fact that the two sovereigns will probably be wearing the insignia of the two oldest non-ecclesiastical orders of chilvary in Europe: the Order of the Garter (which will probably be bestowed on Don Felipe ) and the Order of the Golden Fleece (which Queen Elizabeth already has). That will be quite a sight BTW !