There has been an unpublicised change in the UK regs on this. For many years, a G-reg could be flown on any ICAO PPL, noncommercial, VFR. It could also be flown IFR (if the pilot had an IR) but only OCAS e.g. Class F/G only. This meant, for example, that an FAA PPL/IR holder, flying a G-reg, could fly an ILS (in actual IMC, so we avoid the debate of flying an IAP in VMC ) into Biggin EGKB...