Had dongle and used 15 GB a month every month from October 2008 to Sept 2012 with no problems (most of the time in N Wales, but also in Staffs and on Merseyside). Used in place of a landline when moving.
So happy I had a SIM only deal from Feb 2012 (started at 15 quid, went up with a minor price rise) providing 600 minutes, 3000 texts and unlimited internet. Used 20 to 30 GB a month on the mobile, near enough 600 minutes and a hundred or two texts. They upped the limit to 5000 texts on that plan (Ultimate 600).
If you have reasonable coverage then pricing starts from 6.90/month for 200 minutes, loads of texts, but only 250 MB of data. Really depends what you are after... other networks might have good deals too, such as Tesco, T-Mob (data), Orange (minutes).
I know you can cap the amount you will pay on Tesco and Three, so those might be worthwhile (and means going over the limits won't hit you with big bills - Three has a per MB data charge if you exceed the limit which means 1 Gb costs about 100 quid. It's frightening if you aren't on an unlimited plan!)