I don't know about dunnunder, but signal strength for 3G in the UK is enormously variable, especially in high density population areas. When I go to North London, (Holloway Road area) I seldom get good internet access, but I drove up to Nottingham on Tuesday, and signal strength was good most of the way.
No wonder they say you can have "unlimited" access...........it is unlimited when and where you can get it.