Three does OK in London and in Cambridgeshire, except some very rural areas. Coverage in the fens can be quite good, out there in flatland the masts can be seen for long distances

For prepaid data anywhere not very rural, Three is probably the best deal. £10.49 for 1GB/1 month, £20.49 for 3GB/3 months. I use the latter in my tablet and rarely lack signal when other phones do.

I doubt you can get cheaper than a tenner all in for a week or two data.

You can pick them up at any three store or other general mobile stores (eg Carphone Whorehouse).