....621, the Polish Navy's mine hunter ORP Flaming.
All are taking part in the military exercise, Exercise Joint Warrior 142. This multinational exercise is held twice a year in the spring and the autumn, and the present one involves twenty-two naval units and over fifty fixed wing aircraft - more details on the RN site here. The exercise also explains why we're seeing much more air activity over us, including, today, a low-flying Hercules transport and a fighter.
Conspicuously missing from the parade were any RN ships. They tend to move south later, after the foreign ships are out of the way.