Ofcom have discontinued the Ofcom Sitefiner website.
They now just operate a basic map service to show which networks have coverage indoors or outdoors for the individual networks.
In most cases an Omni Directional Antenna such as the Fullband® MIMORAD® situated outdoors will provide an adequate solution becuase the 4G antenna can be located outdoors where you have a 3G/4G mobile network signal and this can be delivered to the 3G/4G router indoors where there is a pooer signal.
If you need to use a directional antenna then there really are no tools available to find the closest mobile base station and determine which network it is - there will be some trial and error involved unless you contact the mobile networks directly to ask for details of their cell site locations.CLICK HERE TO VISIT THE OFCOM MOBILE COVERAGE SITE
An alternative to the Ofcom Sitefinder is the Mast Data website - http://www.mastdata.com/