Firstly, a radar service will be of NO help with hitting mountains, because the ATCO is not permitted to provide a service below a minimum vectoring altitude, and the MVA will generally be at, and often way above, the safety altitude for a wide area.
Secondly, there has not been a post WW2 midair in IMC in the UK. The sky is huge, and GA activity gets very thin the moment visibility falls below 100,000m and the cloudbase falls below 10,000ft
I did a nice VMC/VFR trip today, in ~ 4000m vis in typical English summer haze and there was almost no traffic about, presumably because one cannot navigate in these conditions using the goode olde PPL-style map compass and stopwatch method
In IMC, the chance of hitting somebody else is utterly miniscule. It's away from Russian Roulette by many orders of magnitude.
And so many targets are non-transponding (the Civil Liberties crowd) that a radar service is worthless on perhaps 80% of conflicting traffic - because there is no altitude information.