The Ofcom Sitefinder was a website operated by the telecoms regulator which would show the locations of mobile phone base stations / masts in the UK. The data on the Ofcom Sitefinder was usually 12-24 months out of date but could provide a general guide about the location of mobile phone masts in your location.
Now Ofcom just offer a generalised map showing UK coverage but without the location of any masts so that if you intended to use a directional antenna then it would have to be installed on a trial and error basis.
Here is the link to the OFCOM MOBILE COVERAGE MAP
There is an alternative to the OFCOM sitefinder and that is the MASTDATA website which shows the details of mobile phone base stations.
If you are a professional installer of 4G routers and antennas or 5G Routers and antennas then you may want to invest in a 4G signal tester such as the CSL Signal Analyser. The CS2389 4G Signal Tester allows you to run a quick network scan at your physical location and view information about all the available 2G, 3G and 4G networks, with base station information and signal strength information so you can make an informed choice about which 4G SIM network you will choose or where the best location to install your 4G antenna.