I'm not aware of any UK "rule" thst says pax cannot sit behind the controls. How would anyone ever take pax in a Cessna 150 if that was the case?
Neither am I aware of rules that stipulate qualifications required before you can touch the controls. A student pilot, pre solo, has no "qualifications" whatsoever, not even a medical cert, so this is clearly a red herring; equally there is nothing to stop you letting your mate have a go either, as long as you are not passing it off as instruction and you (not him) aren't properly licenced for that.
No doubt this pragmatic situation represents the "antiquated system" that oogle so sourly refers to - he's right, he doesn't know! (He's forgetting, of course, that the Aus rules themselves are derived in their entirety from the old - er - antiquated - Board of trade regs from the 40s and 50s...)