I'm back into GA instructing after retiring from airline flying.
You need to:
Renew your SEP Rating. Training as required (at least a couple of hours) + Proficiency Check. Total flight time - I would suggest 3 hours upwards...
Attend an Instructor Seminar. Mine was over 2 days and I really enjoyed it. Book well ahead...
Renew your FI Rating. Training as required (at an ATO) + Assessment of Competence. Total flight time - I would suggest 6 hours upwards...
Consider what to do with your IR (if anything). An IR attached to a Type Rating isn't valid on SEP/MEP. You don't even have IR(R) privileges. I'm not finding it straightforward on how to add an IR(R) back onto my licence. Not sure how IR(R) will resolve after departure from EASA.
A UK National ATPL (now only valid when flying non EASA types) gives IMC Rating privileges whilst ever the licence is valid. It would be nice if, after departure from EASA, our UK issued ATPL's become National Licences again with attached IMC / IR(R) privileges...
Please let me know if you find anything out about this!!