Ryanair UK only has one aircraft, G-RUKA.
Those flights with the RK code are indeed Ryanair UK flights (as needed to continue after 1st Jan), but 'operated by Ryanair' (ie the aircraft is leased in from Ryanair DAC/Ireland).
Whether the Ryanair UK fleet will grow in time I'm not sure, but so long as they're a UK carrier (based in STN) with it's own AOC, there's no reason to my knowledge why it couldn't operate using any aircraft leased in from any one of the other Ryanair subsidiaries.