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 The data on the Ofcom Sitefinder was usually 12-24 months out of date but could provide a general guide about the location of mobile phone masts in your location.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.