Roffa - I had the impression that controllers at London Control, and possibly Gatwick too (i.e. those who rarely if ever provide services to GA especially VFR GA) had some training to understand this.
This is because of another possibility: you can file an IFR FP, under IMCR privileges i.e. no Class A, and if you do it "right" it will not only be accepted by IFPS (which is not hard to achieve if you do it at low levels) but may also be accepted by London Control. And then you get sent into Class A pretty quick. This has happened in years past and confused the hell out of ATC.