Having now re-checked: Mandatory IFR in airspace Class A, B, C, D & E was not allowed but, as you point out, in Class G (&F) it was.
Indeed. As for "mandatory", I've never really understood the phrase "in circumstances that require compliance with the IFR". If you accept an IFR clearance to enter controlled airspace, are you not flying "in circumstances that require compliance with the IFR"? Or is there a suggestion that a UK PPL could accept an IFR clearance in class D in VMC?