Now that the original Brexit day has passed, EasyJet and Wizzair seem to have separated out their UK/European operations. Most EasyJet flights using Luton are now flown with UK registered aircraft and using the EZY flight code. Some flights from EU destinations are operated by Austrian registered aircraft, carrying EJU codes and callsign Alpine.
Similarly, Wizzair UK flights are operated using G- registered aircraft and W9/WUK codes, whereas most Wizzair flights use the Hungarian registered fleet and W6/WZZ codes. There is some crossover, but not much.
All Ryanair flights still use FR code and Irish-registered fleet (not even sure if any aircraft are G- registered).