I've been with Three for over a year on their One Plan one month rolling contract and always been happy with them. I certainly plan to continue to use them for the foreseeable future.
I recently upgraded my phone from unlocked 4S to an unlocked 5 and while I don't think the download speeds are any faster between the two handsets I have always found the 3G to be decent pretty much everywhere.
I've seen a few people posting speedtest.net results as some people find them useful, so I thought I'd do the same. This was taken this morning in in Dorset.
I've just done mine. In Central London. 1.58Mb Down 0.39Mb Up