I'm not sure why I haven't come across this before - the PhoneGnome Box. It looks like it's a standalone SIP based FXO/FXS. It's $99 - I've sent an email to try and find out what shipping to this side of the Atlantic would be.
update - Well, I would get one - shipping is $40, so it's only £70 total, but I can't seem to get them to process my credit card. I know I have enough credit, I know I entered the right details, but I keep getting "Declined - Matched AVS/CVV2 Filter"
update - my bank phoned me up - they noticed that they received several authorization requests at once (that they authorized). I've tried to phone PhoneGnome, but nobody's home. I've sent an email - I'm guessing no one will be in till monday. This is pissing me off.
update (2007-7-24) - I got the order through, but phonegnome had to put the order through manually. Now I just have to wait for it to turn up (but I guess I'm going to have to pay VAT when it does)