Having recently gained my EASA ATPL(H) based on my UK ATPL(H) with Austro Control, as far as I am aware the issue date of your UK ATPL is irrelevant as the process is based on you having an ICAO ATPL.
The important consideration is whether you have to do the EASA ATPL exams or not. I did not as the last type rating and IR proficiency check I took under see EASA regulations expired on 31st December 2020 when we left EASA, which meant my EASA ATPL exams were within the 7 year window from then to be accepted as valid.