If you completed the ATPL Theory under Austro and passed all exams before Dec 2020 then the UK CAA should recognize all your passes as equivalent. You may need to apply to have those results recognized by the CAA (and make sure to do it before Dec 31 2022). You can then do the practical training at any ATO which is authorized by both the CAA and EASA, which would mean doing training once, but you would have to undertake dual skills tests for each element, once for EASA and once for the CAA... if you went to an EASA ATO which was not recognized and authorized by the CAA, then I guess you would be unable to take the CAA skills test, and would have to go through a certain amount of training at a CAA ATO in order to be able to fly the CAA skills tests. So the best bet is to find an ATO that is under both authorities. On the renewal front, I'm not sure, its not an issue I have to contend with... I would imagine that if you can find an examiner who has dual licenses, you'd probably be able to get them to sign off both in one go.
Good luck trying to communicate with the CAA... contact with North Korea is easier!