I'm on three. it's an absolutely terrible network for the most part, but it is extremely cost effective which is why I'm still on it. I pay £10 a month on my phone for 8Gb, unlimited minutes and unlimited texts. They do data sim-only plans for even less. I've really been unable to find anything cheaper. I was on Virgin on my iPad for a while, but they put their sim-only data plans up by quite alot half way through my contract.
I should say that on Three, the coverage was always just fine. In fact internet service isn't too bad. It just happens to drop calls all the time, or I get "call-failed" messages over and over. however, since you only want it for data, you might find you're alright with it.
The other benefit is you can use your allowances not just in europe, but a number of non-european countries too which was quite a draw for me - I used to work away quite alot.