Everyone knows that T-Mobile coverage in the UK is about the worst of all network operators. Even in right in the middle of Central London I know people on T-Mobile who are unable to get coverage.
Top of the coverage list is Vodafone, then 3 .... and then the recently merged Orange/T-Mobile behemoth (once they get their act together and finish integrating their networks, things may improve, but for now they're about as good as each other).