They were a pair of US Marine Corps F-35B (169678 coded CF-05 and 169587 coded CF-02 both of VFMA-211) routing from Lakenheath direct to HMS Queen Elizabeth off the south coast (Plymouth area?).
Earlier in the afternoon there had been two pairs of RAF F-35B from Marham, followed by two pairs of USMC ones from Lakenheath, before the pair you saw at 17:20. Another 8 aircraft (4 RAF, 4 USMC) are expected to join the boat tomorrow to give a total of 18 on board.