Hutchison 3G have been combining their network with T-Mobile's in the UK, and O2 with Voda's - it's mainly an exercise in cost reduction.

So far here in Cambridgeshire it's not affected me much if at all, but then T-M didn't have much coverage outside the city. I would guess that in London there'd be a lot more changes to consolidate base station coverage.

Fundamentally, though, 3G at 2.1GHz is "broken" in that it's not really the best frequency for a mobile service - even GSM at 1.8GHz suffered from the problems of getting good RF coverage, and 2.1GHz is worse - signal absorbtion by trees, walls etc. I'm hoping that operators will be allowed to run 3G services over the GSM spectrum at some point, but it probably won't happen to maintain the stitch-up that was the original massive licensing costs of the 3G auctions.

Oops, went off in a tangent. back on track...

so, with 3G, yes, you'll find it unreliable except near windows and outside.