Coverage v BT

[MSF]

You can start by taking a look at the coverage checker -  https://smarty.co.uk/coverage-checker/

You will then need to do the same for BT.

That said, a coverage checker is unlikely to give pinpoint accuracy for the places where you use your phone most. As an example, you may work in an office where there are a large number of metal beams that interfere with the signal from one mast more than another - a coverage checker cannot tell you about this.

Friends, colleagues and family may also be a good source of info as to coverage in an area

[Chalkychap]

Online coverage checkers can only ever give you an approximation of actual coverage in any given location.

The coverage maps are produced using computer models, which are all based on certain assumptions that can mean theoretical coverage and actual coverage are sometimes worlds apart for any given location.

As @MSF says, your friends and family can also be a good source of info as to coverage in any given area.