I remember (wearing another hat) being bullied some years ago by someone who kept telling me I had to let them do X because that was the law. I kept asking to be shown the law, or for a reference to it. None was ever provided.
Eventually, I asked an expert lawyer myself. No, he said, there is no such law. Your answer "No" to their request made it unlawful for them to do X. (X was to do with gravestones).
I've not seen any law on cellphones in GA aircraft in the UK, or any reference to anything that fits. Maybe it is forbidden, maybe not. I just don't know.
I don't plan to use my cellphone in the aircraft anyway - under IFR certainly not, for reasons set out above. I fly with two COM sets and a handheld (which IS covered in the aircraft radio licence).