Maybe you can go with Three (3)? Do they still have the largest 3G network in the UK (you mentioned streaming so you'll certainly need 3G)? They seem to have reasonable rates if their level of 3G coverage really is as large as they claim it to be. Others people have told me that 3 sucks due to their practice of outsourcing customer services, though I believe one should almost never need to speak to CS with a good operator.
Luckily as you're in the UK you have your pick of the litter as far as which operator to go with.
