I have an O2 dongle £10.48 per month for 1gb or £15 for 3gb. It is just pay as you go. You don't have to top up each month. I put credit on it yesterday time before that was Christmas.
I use the Three network when I am in my 'van in Cornwall, via a dongle. If you go to the Three internet site, you can put the postcode of your caravan site and see what the coverage will be.