It's an IRC channel called #bitcoin-otc-uk (they have ones for the usa etc too). I just talked to people and found someone who wanted to buy. They have a little rating system and someone who was online said they'd dealt with them before. So that was good enough for me as they had good reviews and seemed trustworthy. I then transfered the bitcoins and they sent the money by bank transfer. (We made the transaction publicly in the chatroom so that there'd be others to record it.) It wasn't too bad. If you want to buy/sell some I'd recommend this.