I have a T-Mobile dongle on £15 p/m PAYG. The connection speed is slow (1Mb/s on average), but it's fair-use policy make it a clear winner if pure speed is not your primary concern.
Once you download over 2Gb in any one month, they will try to stop you streaming media and downloading large files between 4pm and midnight, but it's not that much of a problem.
I can easily download large files and even persuade it to use bittorrent, and can download more that 10GB in a month, with no extra charges.
I agree connection and coverage can be patchy, but unlimited downloads for £15 per month; I'm not complaining too much!