Consider the size of the UK vs the US. We have national networks here, not regional.

There are differences in coverage, but they won't affect you in the areas you'll be in. Traditionally, O2 provides the best rural coverage and in fact I'll always be on O2 because I end up working a lot in the countryside. Where I used to live in North Yorks, everyone was on O2. It's the difference between 95% of the population (most of the networks) and 95% of the UK (O2) in how they express their coverage. But since you'll be in urban areas, you won't need to bother about that difference, and should get pretty good coverage on all networks.