In the U.S. as well, in uncontrolled airspace, you can fly IFR/IMC without a clearance.
I understood that all airspace in the USA above 1250ft was class E therefore controlled for IFR. Have I misinterpreted that?
I know it's legal to fly IMC without a plan in the UK, however I don't consider it very smart, so yes, I'd probably still call them a clown or something similar.
There are airlines in the UK regularly flying in class G airspace IMC without radar cover (only procedural service). I don't see what protection a fight plan can offer in these circumstances.
D.O.