I read the other day... OCAS: 4 mid-airs in the UK in past 10 years, all below 1000ft in VMC. Last mid-air in IMC was in WW2. This is probably due to the far lower traffic density in IMC, together with IMC traffic being generally above 1000ft. Certainly, there is no case for worry about flying in IMC without a radar service - it's by far the safest place to be.