Indeed Ryanair could've re-registered their entire UK based fleet to G- aircraft, very much like how Wizz Air UK have done with everything based in the UK using the W9 code on G- aircraft. Then of course they wouldn't be restricted as to which routes they fly (ie a handful of LPL-RAK a week amongst an entirely UK-EU route network wouldn't cause any issue).
The thing is, Ryanair aircraft and crew in the UK are employed on the Irish (EI-) register along with Spain, Portgual, Greece and I think a few others. Those in Germany, France and Italy are on the Maltese (9H-) register and likewise Poland, Czech, Hungary etc. on the Polish (SP-) register. All comes down to labour union and tax cost savings to the Ryanair "group". Wizz are not represented by trade unions at the moment. When this inevitably happends at some point, they'll start to lose their cost advantage over Ryanair on labour terms and similarly may end up creating separate registers.
If, as Skipness suggests, you had everyone in the UK move to the G- register, you'd need to abide by UK employment terms as well as potentially reduce your fleet agility with these aircraft not being able to move as freely around the group network since they're registered to Ryanair UK, outside the EU. With only a handful of UK-Dom/Non-EU routes in the Ryanair network, it's perfectly understandable why they haven't gone down this road. It's totally unneccesary for the other 95%+ of the UK route network that serves the EU and can continue to operate on the current register.