They would more than likely just issue you with a vehicle rectification order so you'd have to go have an MOT redone. If it failed on emissions due to a lack of cat then obviously a cat would need to be refitted to pass then you take the MOT certificate into the old bill.
If the car was taxed MOT'd and the mods were declared (they had been told it had an aftermarked exhaust fitted) then I dont see how an insurer could argue on paying out tbh.
If the car was taxed MOT'd and the mods were declared (they had been told it had an aftermarked exhaust fitted) then I dont see how an insurer could argue on paying out tbh.