Ofcom (the telecommunications regulator) has provided a mobile signal checker. The service allows you to check your location for the availability of voice, 3G and 4G services from the four main UK network operators (EE, O2, Three, Vodafone).
Since all the other mobile phone operators in the UK are provisioned over one of the above networks, you can use the information to find out coverage for any mobile operator once you know which underlying network they use.
The service provides coverage for locations both indoors and outdoors, so it's very useful if you're looking to switch to another mobile network and want to check if you'll get signal coverage in your area.
Ofcom uses signal level predictions provided by the UK mobile network operators, so the results should be quite accurate, and they will continue to conduct testing with a view to improving the accuracy.