Probably obvious, so sorry if you already did this, but have you taken a look at
www.britcoin.co.uk.
So far as I know their code is open source, but either way you could talk to them.
Its a working exchange which I have used, and seems to be clear of a lot of the hassle others report with othe exchanges, although that is possibly down to the people running it or what the the law here in the uk allows.