Harry,
What phone do you use? I ask because you could use tethering instead of a dongle.
I previously used GiffGaff. I found that the 3G signal was pretty weak and the connection would always die. Switched back to Three on there £15 All you can eat data PAYG. Not only do I get good speeds, so long as you stick to under 80 GB's a month of data I've had no problems Tethering.