Exam validity for an IR is treated in the same way as for licence issue and for a candidate who completed their TK exams with a non-UK ATO before 31st Dec 2020, the validity for a UK FCL IR issue would be 36 months or until 31st Dec 2022, whichever was sooner, as stated above. However, your case is different as you intend to complete your exams with a UK approved orgainsation post-31st Dec 2020 and there may be scope for granting the full 36 months from the date of successful completion of the last TK exam on the basis that this will be the trigger date for the validity period, provided your earlier 'EASA' credits are still within the 36 months.
I would suggest you contact your UK ATO for definitive guidance here though.