My experience as a field engineer... (your mileage may vary)
London - Any network is good, o2/Voda/Orange (voice), h3g/tmo (data).
South of London (M25 to coast) - o2/orange (voice), h3g (data). (big dead spot for Voda A275 area)
South West (M4 to coast) - o2/Voda/orange (voice), h3g (general) / Voda (M4 corridor) (data).
Devon/Cornwall - Orange (voice), h3g/orange (data).
North Wales - o2/voda/h3g (voice), h3g (data)
South Wales - o2/voda (voice), h3g (data)
West Scotland - o2/voda/h3g (voice), h3g/orange (data)
In general, o2/voda seem to have voice covered.
Notable exceptions:
Voda south of London (A275 area) is very lacking in coverage.
Orange gives more signal (not 3g) in more places in Devon/Cornwall than any of the other operators. Very good coverage out of towns.
H3G seems to do particularly well along the North West coast of Scotland and down into North Wales. Even for voice!
For data H3G is by far the best, with much better 3G signal coverage and generally better transfer rates where the competition achieves 3G. TMO comes in second.
The Orange/TMO merger may change things, but there is a fair way to catch up.
Voda/o2's 3G coverage is a joke outside of major populations, and fairly slow within them.