Hi D-D-T,

I live in the US and can't really answer your question directly--partly because I don't know where in the UK you happen to live.
That being said, if you live anywhere near London, I really doubt that you'd have much difficulty finding someone to ink your bottom.
I say that based on my frequent visits to London and viewing not only the various venues of the city, but also the denizens, who often display some of the most interesting (outrageous?) visible piercings and visible tattoos.

Good luck in your quest.  I would do some internet searches on your local/nearby tattoo establishments.  Often you can tell a lot by reading the short bios of the artists and viewing their portfolios.