I must be the unlucky one. O2 were all over the place for my data so I swapped to 3. At times I think 3 are worse. London Bridge is terrible, my GS4 often misrepresents the quality of signal, Hampstead is even worse than London Bridge, signal indoors is patchy, coming back into an area with signal (e.g. inside to outside, underground to surface) the signal can take quite a while to register with the network. I haven't tested the speeds recently but they weren't that impressive. My 3 in London experience is rather disappointing.
Inside my office (not too far from a window) now with full signal (H+); 88ms, 1537kbps down, 60 up.