I think the main problem associated with flying in IMC outside CAS in the UK is, as been inferred above, the lack of reliable radar cover.
Perhaps one of the few areas of UK life not under constant surveillance from nuLabor's nanny state goons!
It is not illegal to fly IMC without any radar cover or even without talking to anyone - but it's perhaps rather foolish. Hence the attraction of lower level airways to those appropriately qualified flying suitably equipped aircraft.