Mobile phone store in London - need unlocked quad-band GSM + UMTS or HSDPA

[PhotoJim]

I'm going to be in London for ten days as of next weekend and I'm trying to find a reasonably-priced quad-band GSM + UMTS/HSDPA 2100 phone (even better if it has UMTS or HSDPA 850 and 1900 too but that's probably asking too much ) to use as my travel phone and a plaything back home in Canada.

I'm going to be using it with several providers so it needs to be unlocked. I know several carriers sell these at decent prices with prepaid SIMs, but I assume they'll be SIM-locked, and even if they aren't, I assume they won't be easy to get working for data access with other providers. (Two of my existing 4 phones are SIM-unlocked but I can't change the data configurations, which is slightly annoying. The other two are unbranded so I'm good.)

Suggestions of stores? Suggestions of phones to look at? I couldn't care less about camera functionality. It'll get used as a phone first, and as a data tool second, and anything else is bonus.

I'll also be in Zurich, Edinburgh and Glasgow if you know any good stores there, but we're in London first so that seems the best choice.

Thanks!