Details of the BBC FM transmitter networks here -
Mast heights here -
Map here -
The locations are given as Ordnance Survey grid references. There's an online converter here - gps.ordnancesurvey.co.uk
Though this one http://www.movable-type.co.uk/scripts/latlong-gridref.html is perhaps more useful in this case.
For CI and NI need this code -
Les Platons, Jersey located at 49.2477N 2.1049W
See Google Maps example here -