I still don't get it. Prince William was born into the royal family, thus he would have the title HRH. Since in his royal highness status, why was he a commoner?
There are three categories of people in the UK:
1. The monarch - he isn't the monarch and so can't be category 1.
2. Peers of the realm - he didn't have a substantive title until he was created Duke of Cambridge so he wasn't a peer of the realm - he couldn't have sat in the pre-1999 House of Lords until he was created Duke of Cambridge so he can't be category 2 which only leave category 3 - a commoner.
3. Commoners - as he wasn't either of the above he was a commoner. He was eligible to vote for and to stand for election to the House of Commons. Kate still can by the was as can Harry, Beatrice, Eugenie, Sophie, Anne, Birgitte, Katherine Kent, Michael and Marie Christine and Alexandra - all HRHs but all commoners.
I remember way way back when Britain first allowed 18 year olds to vote and there was a story in the paper about Princess Anne being eligible to vote in an election and she was asked about that and said something along the lines of :I am eligible to vote but like the rest of my family won't do so.
Queen Victoria often objected to being told to make her sons Dukes but when it was explained to her that if she didn't do that they could vote and stand for election to the House of Commons because they weren't peers of the realm she immediately agreed that she needed to promote them to the peerage. She didn't have to worry about her daughters as women weren't allowed to vote in the UK until 1918 (over 30) and 1928 (over 21 - the same age as men). It is for this reason the the younger sons of Kings have been made Dukes - to keep them away from the frontline of politics which they would have to have been involved in in the Commons but in the Lords they never needed to even turn up.
Interestingly if they were living in Australia they wouldn't have a choice as it is compulsory to vote in all elections here (and you will be fined - and even have to fill in forms and produce death certificates to get out of paying a fine - as we had to do for my father with the local government elections last year when the government sent him a fine notice nearly 12 months after he died).
The simple question to ask yourself is: Is this person able to sit in the pre-1999 House of Lords? If the answer is No, unless they are the monarch of the day, then they are a commoner.
So even though William was born a Prince of the realm he wasn't a peer of the realm until created Duke of Cambridge and so until that day he was a commoner.
The bit jump made on his wedding day was not made by Kate who went from commoner to commoner but William who went from commoner to peer.
I think there is some misunderstanding as to who is actually a peer and who is a commoner amongst many people because the Europeans do/did things differently. In Britain only the title holder is a noble - not his wife or his children. As William didn't hold a title he wasn't a peer and thus wasn't a noble. The only alternative is to be a commoner - a royal but still a commoner.