These are the repeaters I use in the Guildford area. I mostly monitor either GB3GF or GB3FN. I can easily get into the Amersham and Harrow repeaters from here, but I don't use them much. During lift conditions it gets crazy - I've been able to get into GB3PY (Cambridge) with no issue running 15W into my homebrew quarterwave groundplane in the loft.
For UHF FM I use 20W into a homebrew quarterwave ground plane in the loft.
The RSGB Emerging Technology Co-ordination Committee (ETCC) does frequency coordination for repeaters, beacons, etc in the UK in cooperation with OFCOM. Plenty of information on their website here. Under the Bands menu, repeaters for various bands can be listed and sorted. I find sorting by distance to be the most useful.
Something I did not realise until fairly recently, is that all beacon and repeater licence applications are done through the ETCC and not OFCOM. The ETCC then interacts with OFCOM. See the Forms section of the ETCC website.