Take care if you're buying from Asda. I've frequently mentioned (as Chuck has just done) that all of their phones are unlocked to all networks. That statement was based upon their in-store advertising material and upon what was shown on their website. But a couple of weeks ago I noticed that their Stowmarket store was displaying some phones on specific networks (which I assume they're locked to). When I next called into their Ipswich store I looked at where the display used to state that all phones sold by Asda could be used on any network. That information is no longer displayed. Further, I can't find it on their website any longer.
It looks like all of the phones Asda sell online are still unlocked but some phones they sell in-store may now be tied to specific networks. However Asda is probably still a good place to try.
Alternatively you can buy a used (tri-band) Nokia 6100 (tested and with 3 months warranty) for £19 here:
http://www.phatphones.com/phones_to_clear.htm
Chris