I've used my network provider in the past, give them a call, and depending on how long you've had the phone they might even do it for nothing. I got a Samsung Galaxy unlocked for £10 by '3' and my iPhone 3GS was unlocked by O2 for free.
There's a place on Cranbourne Street & also there's a guy in the newsagents in North Street, opposite the entrance to the Lanes which a mate of mine recently used
Got mine done in the place down the steps from Churchill square opposite where the old punk T shirts place (Still can't believe that's gone!!) Fiver I seem to remember!
Got my Iphone 3gs done in that little arcade of shops where Panasonic is in town, near M&S and the coffee shop (starbucks?). Cost me £10 but the place mentioned above wanted 15, orange themselves wanted £30.