Vmc & Imc
There is no such thing as inavoidably entering IMC other than owing to a lack of pre-flight planning or failing to look out of the window.
If IMC conditions are entered then undetake a 180 Rate1 turn or descend if it is safe to do so.
IFR or VFR has nothing to do with ATC but are mearly the rules under which you have chosen to fly. If the visibility is less than VMC (<1500m and the relevant closeness to cloud minimas) then you must fly in accordance with IFR whether you are talking to ATC or not. A UK issued PPL will further restrict the pilot to a minimum of 3000m in flight visibilty.
Your next point is obvious (or should be). If the road speed limit is 40mph would you ask if the police frown on the speed being exceeded or if the Dept. of Transport are comfortable with your speeding.
You are either VMC flying VFR or IFR or IMC flying IFR and there are no ifs or buts.
The Certificate of Airworthiness will state whether the aircraft is approved for IMC flights. If it is certified for VFR only then you should remain so.