Three UK has just launched a
new prepaid tariff. It charges 1p/MB for data, 2p for UK SMS and 3p/min for UK calls. With every top-up, you get 150MB of data for free. If you click on the above link and then "
See how we compare", there's a very useful comparison of most of the UK's prepaid tariffs.
I know several Three customers who frequently experience coverage problems, even in London, but they say that when they do have coverage, the quality of the data connection is excellent.
Three's charges for international voice calls remain extortionate though, for example 56p/min to the US.