"You needn't be knighted to change your name to 'Sir' or 'Lady'."
You can't change your name to any title (nobility, religious or social)
"Name restrictions
We will not accept an application for a name that:....
..... may result in others believing you have a conferred or inherited honour, title, rank or academic award, for example, a change of first name to Sir, Lord, Laird, Lady, Prince, Princess, Viscount, Baron, Baroness, General, Captain, Professor or Doctor etc."
http://www.ukdps.co.u...trictionsOnNames.html