This topic was covered on this forum in September 2019. Here is a link to the thread:
Holdover claim revocation - Trusts Discussion - The Trusts Discussion Forum
I agree with the consensus in that thread, that gift holdover relief claims may be revoked, either within the time limit for amending the self-assessment return in which the claim was made (if the claim was made as part of the return) or otherwise within 12 months from the claim being made (TMA 1970, Sch 1A, para 3(1)(b)).
Mark McLaughlin