Thanks everyone. I ended up deciding to just go the two-SIM route. Grabbed one from e-plus here in Berlin (as it was cheap and I won't be using it that much) and will pick one up from one of the providers in London when I return there tomorrow. That is, if I can ever find a cell shop in London that isn't closed! (Saturday and Sunday are probably poor sample days...)

Oh, and my Cingular V600 is, quite conveniently, unlocked. That may be the first time a cell company has told me something good that turned out to be true . . .

d.