I assume you are using Princess transfer to the airport (?). On our last cruise with Princess a few years back, we had a 2.05PM flight the bus arrived at Heathrow (Delta) before 11AM, it was on Saturday with no traffic problem. With a cruise this close just keep an eye on your document/app you might need to make a trip to customer service to make sure you are the first few bus to leave the ship.
Enjoy your cruise and best of luck.