With an EASA licence and UK passport at Ryanair, you could be based in the UK, Ireland or Morocco flying an EI-, SP- or 9H aircraft without issue.
The short answer to your final question is it isn't quick or easy. You need to convert your license from EASA to UK CAA before you can apply for BA, easyjet, TUI etc which is not a quick or easy process.
If you are the stage before getting any licence, you might want to consider getting a UK and EASA licence to keep most options open. This is true if you go for Ryanair or any other path. The additional cost and difficulty is small given the overall cost and difficulty.
If you are dead-set on the Ryanair programme, you are wrong on none being dual or UK CAA licensed. FTE Jerez offer a (the only) joint UK CAA/ EASA Ryanair one. But you likely need a lottery win to pay what they are asking.