Until he lit up, the fact that he was a plod was indiscernible. But if he was right you too should have let the faster car through.

If he had switched his lights on during your passing manoeuvre then, yes, you should have made an effort to get out of the way, abandoning your overtake if necessary. As he was not showing any light or sounding a siren until after you had completed the pass then he was just another road user. He was not right - he should have allowed you to complete your legal move and then taken his turn.

DC