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!
GSM: CA: SaskTel (postpaid voice/data,,postpaid data), Rogers (postpaid data); UK: Orange, 3; CH: Swisscom; AT: tele.ring; US: T-Mobile, AT&T, Red Pocket Mobile, Telestial Simple Calling; EE: AirBalticcard
CDMA: US: PagePlus
Hardware: iPhone 4, iPhone 3G, Nokia E63, Nokia Nuron, Novatel Ovation MC950D & MC998D data sticks, Huawei UG1691 data stick (3G AWS), Motorola RAZR V3, Sony-Ericsson K610i, Sony-Ericsson Z310i, Otech F1 (quad sim) (all unlocked)