I'm surprised nobody picked up on this, as Cornwall is an English Dukedom. It is conferred automatically on the male heir of the Sovereign alongside the Scottish Dukedom of Rothesay (which has the subsidiary titles Earl of Carrick and Baron Renfrew). The male heir (current the Prince of Wales) is also the Great Steward of Scotland and Lord of the Isles.
It was originally conferred to Edward, The Black Prince. But a charter written in 1421 means it is automatically inherited from the Sovereigns eldest son to heir. Which means if William passes away when his father is King, Harry would not become the Duke of Cornwall.
It requires an Act of Parliament to create him Prince of Wales (and Earl of Chester).
As Iluvbertie has said, you are wrong in thinking an Act of Parliament is used to give the heir apparent thr Principality of Wales and Earldom of Chester. It is a letters patent.
The DofE question has been brought up many a time.
If the Dukedom was granted with new subsidiary titles, then Edward could hold a Dukedom, two Earldoms, a Viscountcy and a Barony!
In this case it's probably more likely that he will recieve the two subsidiary titles Earl of Merioneth and Baron Greenwich. And still keep the title Earl Of Wessex.
I'm surprised nobody mentioned that in the Channel Islands she uses the style Duke of Normandy, and on the Isle of Man the style Lord of Man. The former is still used in the Channel Islands as they were part of the Duchy of Normandy.
She is not the Duke in a constitutional capacity and instead governs in her right as Queen. This notwithstanding, it is a matter of local pride for monarchists to treat the situation otherwise: the Loyal Toast at formal dinners is to 'The Queen, our Duke', rather than 'Her Majesty, the Queen' as in the UK.
The Channel Islands are not represented in the UK Parliament but each island has its own primary legislature, known as the States of Guernsey and the States of Jersey, with Chief Pleas in Sark and the States of Alderney. Laws passed by the States are given Royal Assent by the Queen in Council, to whom the islands' governments are responsible.
Whilst this was answered by branchg in post #94, many people forget though that Angus Ogilvy declined an Earldom on his marriage to Princess Alexandra of Kent.
You are right The Queen had offered Ogilvy an earldom on his wedding, which he declined. He also rejected a grace-and-favour apartment at one of the Royal Palaces.
Camilla is officially HRH The Princess of Wales, though because of its association with Diana she uses the title HRH The Duchess of Cornwall. In Scotland she is HRH The Duchess of Rothesay, and Diana was known as that in Scotland as well, though of course to the rest of the world the Wales title is the one that was used.
Camilla's full title: Her Royal Highness The Princess Charles Philip Arthur George, Princess of Wales and Countess of Chester, Duchess of Cornwall, Duchess of Rothesay, Countess of Carrick, Baroness of Renfrew, Lady of the Isles, Princess of Scotland.
The only difference between that and Diana's title was that she had "Great Stewardess of Scotland" as well. It was easier to call her Princess of Wales, as that is her highest rank of title.
It has been stated however that she will be styled Princess Consort. This was one of the reasons for the abdication of Edward VIII because there was nothing to stop Wallis Simpson becoming Queen Wallis if she married Edward as King. Camilla will not be styled Queen though.
There was no way, that Wallis Simpson could of married King Edward and he still remained King. So if he wasn't King, she could never call her self Queen.
The abdication was because Edward had to choose between Wallis or the Crown.
I think it likely they would be granted Dukedoms on any marriage for either of them. In part this would be because if not then their wives would be styled HRH Princess William and HRH Princess Henry respectively!..
This leaves. Cambridge and Sussex as Royal Dukedoms that could be conferred. Sussex might be favoured for William as it is probably the least senior Royal Dukedom that has been used, so the fact it will be in time be joined (and superseded) by other titles might make it a choice.
Whatever dukedoms the princes are given their wives will always be HRH Princess William or Henry. They would just have add ons.
There is a specific thread discussing the titles the boys and their wives may receive:
http://www.theroyalforums.com/forum...of-william-harry-wives-and-children-6755.html
I presume Harry will become Duke of Sussex. And William may or may not be given anything.