This is my first visit to this forum for some weeks. It reminds me why I don't come here often.
I've forgotten to turn off my mobile phone before flying a few times. I soon find out: as I pass 1000 feet or so, it loses signal and starts its "mating call" which I hear in the headset. Finger goes to top pocket, presses the "power" button and holds it for a couple of seconds, and the noise stops.
An earlier cellphone of mine couldn't be turned off in that way, so it stayed on during the (short) flight. On the ILS (in VMC, practising) in the Arrow, it burbled in my headset and at the same moment the localiser needle swept full-scale from side to side (rather like a windscreen wiper). There are several different signal paths the interference could have take to cause that, but it's irrelevant. That cellphone interfered with that ILS display. I will not fly with a cellphone switched on in the aircraft. I will certainly not fly an ILS with a cellphone switched on in the aircraft.
Despite all the bluster, I've not seen any law that says "thou shalt not use thy cellphone in a GA aircraft in the UK". If there is one, it would be useful to see the reference. Since cellphones don't need a licence for "normal" use, it would probably need a lawyer to interpret the regulations.
In the USA, use of cellphones in GA is allowed. I've been told (I've not checked it) that selected cellsites even have an antenna set angled upwards to facilitate them. But that's the USA, where they do lots of things differently.