from memory, the most common form of Midair collision in the UK is one in the visual circuit...
I think historically there have been some midairs over the years between aircraft enroute in Class "G" or the open FIR as it was once called but again the majority have occurred in VMC.
The intersting point about IFR in class "G" is that the rules make no differentiation between VMC and IMC if the flight is being made above the transition level. In this case all that is legally required for IFR is that the pilot complies with the Quadrantal rule for level allocation.
Gliders are another case - I believe that for a a glider to fly IFR all that is required is that the pilot wears a prachute.
As a previous poster said - it might be legal but would you want to do it?