Data: http://maps.ofcom.org.uk/check-coverage
This set for example just shows 4G data and indoor, as that would indicate the best signal in terms of penetration and speed. Note the saturation has been increased to help distinguish the difference between white/green as they were very faded in the original.
The data seems to line up fairly well with a study I did 3 years ago:
Vodafone - 80%
O2 - 77%
EE - 68%
3 - 51%
