It is not illegal to fly IMC without any radar cover or even without talking to anyone - but it's perhaps rather foolish.
It appears foolish but the IMC midair stats (zero, zilch, none, nil, since WW2) do not support such a fear.
Hence the attraction of lower level airways to those appropriately qualified flying suitably equipped aircraft.
Sure, you need an IR etc. But it doesn't work too well in the UK, especially the south. The lowest levels at which one can get LTMA crossings are around FL100 plus. People who succeed in filing a Eurocontrol routing at FL070 or so (and it is possible) end up either vectored all over the place, or get dropped out of CAS at the first convenient (to ATC) opportunity.
Also if you fly at say FL070 you don't have to go very far away from London to find yourself in Class G, and then the service from London Control will most likely be terminated, your IFR clearance is in the bin, and you have to continue OCAS.
As many pilots have discovered, you can fily flight plans via Eurocontrol even lower e.g. FL050. These are meaningless and end up in meaningless flights - forced below CAS, probably from the start.