I believe his full royal styles are HRH The Prince of Hanover, Duke of Brunswick-Luneburg, Prince of Great Britian and Ireland. In Germany, legally, he is simply Ernst-August, Prince von Hannover.
In the UK, he would be HRH Prince Ernst-August von Hannover, assuming the Queen recognized his rank as a Royal Highness as an extended member of the royal family (which is likely).
In the UK, he would be HRH Prince Ernst-August von Hannover, assuming the Queen recognized his rank as a Royal Highness as an extended member of the royal family (which is likely).