Can't think what gave you that idea! Anyone would think Khan had the chance to also capture the British Title but withdrew ....
I dont think you can UNIFY them, but you are certainly allowed to hold both titles at the same time, as Junior Witter and a fair few others have done. Also, the BBBC, Commonwealth and EBU aren't like the IBF and WBO, forcing people to fight **** mandatories, and are proud to have the better boxers holding their titles - the alphabets give lip service to certain promoters, as everybody knows.
No doubt you could try - but I think most of these trinkets are given as part of a 'major' title fight. Examples include Mayweather picking up an IBA title, Hatton picking up the IBO etc. The WBU is controlled by Sports Network, whilst the WBF is won outright deliberately - if you get my drift.