500e - to clarify further to others who've already posted with technical details - the main TETRA head which controls all the Police radios is lost when both Gens go offline, in fact, thinking about it now, since TAC 4 also goes through the same head, I'm pretty sure it would also become unusable, even though the actual radio would still be powered by its own battery.
In addition, if the Pilot happens to be using ATC1 in a twin Garmin 430 fit, typical of UK Police-fit aircraft, that will also drop offline when both Gens are lost (as would happen when engines are chopped). In a fraught situation, he's unlikely to be able to do anything other than press the button & transmit - transmission goes out, or not, through whichever radio was last selected. With a double engine failure, it may or may not be ATC2.
Transponder - forget about it, fingers don't even go anywhere near them in day to day work, as it lives with the same code 24/7.