but signal strength for 3G in the UK is enormously variable, especially in high density population areas
It also varies enormously by which network you're on.
For example, someone at work is an arms length away from me. She is on 3, I am on Vodafone. Can she get more than a couple of bars worth of signal, let alone 3g ? Fat chance. Whilst I get full signal and 3G, no problem.
T-Mobile are another network that have a particularly bad reputation in London.