Vote for 3
I've had two 3 mobile broadband sims for 2 years now and recently extended both contracts (each are 1Gb per month for £5 inc VAT - yes JUST a fiver for each!) and was "given" a new (faster) USB dongle and a MiFi dongle for nowt.
One of my original dongles is now fixed in the car linked up to a netbook acting as a trip computer/GPS/OBDII/webcam/wifi router ... and it gets a good signal most of the time and holds a connection very well even when driving around the countryside.
I've been VERY happy with the 3 network and have no problem recommending them and it sounds like they can only get better. The tariffs are very reasonable and the more you buy the better discount you can get 8-)