If already a Three mobile (pay monthly) customer, there is an unbeatable deal, half price on all mobile broadband packages, maintained as long as you keep a phone contract)
http://www.three.co.uk/personal/prod..._customers.omp
1GB/month = £5 /mo
3GB/month = £7.50 /mo
7GB/month = £12.50 /mo
Don't go to low, as it's a whopping 10p/MB beyond that - so exceed the 1GB by just 50MB and you've spent another £5