I like to admit it when I am wrong and following more research I must admit I made several incorrect assumptions, statements, etc. in the preceeding posts.

My apologies to all!

I did find that Prince Charles holds titles that could be used by Prince William as courtesy titles (although he chooses not to do so), such as: Earl of Chester, Earl of Carrick, Baron Renfrew.

But more to the point I also found information summed up as follows:

In British law, a commoner is someone who is neither the Sovereign nor a peer. Therefore, any member of the Royal Family who is not a peer, such as HRH Prince William of Wales is a commoner, as is any member of a peer's family, including someone who holds only a courtesy title.

But also, traditionally, members of the House of Commons were commoners and members of the House of Lords were peers, although peers whose only titles are in the Peerage of Ireland have been able to stand for election to the House of Commons for centuries. Since the House of Lords Act 1999, which excluded most hereditary peers from the House of Lords, most hereditary peers can now stand for election to the House of Commons, even though they are not commoners; this is not a contradiction because the word "Commons" in "House of Commons" does not refer to "Commoners" but rather to an archaic word for "communes", i.e. communities. For example, the 13th Marquess of Lothian (aka Michael Ancram), the 3rd Viscount Thurso (aka John Thurso) and the 3rd Viscount Hailsham (aka Douglas Hogg) are or have been members of the House of Commons.

So apparently since 1999 even Prince Charles could run for office as an MP.

A note of interest and humor; Princess Anne,The Princess Royal, has chosen that her son and daughter do not have any titles to help them have a more "normal" life (despite the fact the Queen has offfered to give them titles).

P. Anne has been quoted as saying that her children are not Royals, their grandmother just happens to be the Queen