The UK CAA did say that they would recognise EASA certificates for up to 2 years. Recently, however, they have backed out of this and said that they will only recognise certificates 'current and valid' at the end of the transition period. In other words, previously you could plan to take EASA CPL IR flight tests after Jan 2021 and have it added to your UK licence, now you can not. Billie Bob has explained why UK ATOs are setting up the insurance of either a direct EASA approval or approval through (in) an EASA State. I can assure you these commercial decisions are not rushed, but are carefully considered. I do not agree that it would lead to complications.
IMHO the UK CAA have shot themselves in the foot by failing to consult properly with ATOs before they make decisions, but we have learned not to rely on them.