Hi Kristian - while I cannot give you a concrete yes or no on this question, a good place to start would be
www.royalmail.co.uk - they have a list of restricted items page on there somewhere mate. There may be no direct info on posting soiled panties to UK postal addresses - but I would have a peruse of the section related to obscene publications - which will be on there for sure.
I think this is a bit a of a gray area really - but what I can say is this - the market for this kind of stuff is pretty big - and I'm sure there are tons of soiled panties working there way up and down the UK in mail trains as I type this ;-)