I recently attended a CLA (Country and Land Business Association) Rural Broadband Conference down in Winchester. There were a number of providers exhibiting.
If you PM me your email address, I'll send you some of the presentation slides we were sent.
Have you looked at Satellite Internet, I can't see you being much of a gamer with your current setup, so latency shouldn't be too much of a problem.
TooWay Satellite has 10/20/30 GB allowances, with the 20/30 plans having a 23:00-07:00 night-time unlimited allowance. They also have an "Absolute" plan which is totally un-restricted.
They quote 20Mbps down/ 6Mbps up.
Prices for the above 4 plans were (roughly), £25,£35,£45 and £65
(for the Medium/Larges/X-Large/Absolute)
was the link of the people presenting.
Other satellite providers :
I know you say you are using a 3G aerial, but have you seen the WiBE(r)
Basically it's a high gain aerial unit (with multiple antennae)
There is also: http://www.wiber.co.uk
(which is actually http://click4internet.com/)
(Am not entirely sure if wiber.co.uk uses the WiBE(r) units or if that is just a naming coincidence - I didn't stay for that part of the presentation !)
Hope that can be of help,