“I don't think the powered planes that truly have no electric power available for accessories represent a significant hazard to other traffic. They tend to fly at very low levels.”
You really are out of your depth on this. Lots of home built aerobatic types have no electrics and fly just as high as you. I suppose I will now get told that home built aircraft are not allowed to do aerobatics…
Rod1
PS the mobile phone device was put forward by our very own CAA as a solution to this very problem. Unfortunately, nobody has been able to build one which was rather "inconvenient" for the CAA.