I don't know about the UK, but I know that the Dutch chapter secured a content donation from their parliament of the official portraits of the whole government as a job lot. That might be easier than trying to get them one by one. I also heard that the German chapter was going to try and get photos of all candidates in the lead up to their next elections.
Amusingly, I tried to get the Federal Parliament of Australia (via the Australian parliamentary library) to donate their official portraits of the government along the lines of the dutch model. The request went up the food chain to some anonymous bureaucrat in the parliamet from whence the response came back down that, No, we could not release the official portraits under a cc-by-sa license as that would encourage the usage of those photos to satirize the politicians.
A more ludicrous reason why a free-license can't be used I have yet to hear!
-Liam
wittylama.com/blog
Peace, love & metadata