There are no provisions for those electors with a postal vote at a particular poll to cancel their postal vote. However, they may change their voting method by submitting a fresh proxy application by 5pm, 11 working days before the poll, which would supersede the earlier application for a postal vote.
Postal voters with a longer term postal vote arrangement may cancel their postal vote at any time and up until 5pm, 11 working days before a poll to be effective at that poll.1
However, since postal ballot papers may be issued to electors as soon as practicable, an elector who has received their postal ballot and has returned their completed postal ballot paper for the poll will not be allowed to cancel or make any changes to their absent voting arrangements so that they take effect at the poll (except if the ballot paper has been returned as spoilt or lost).2
This is also the case for an elector voting by proxy, where their proxy has a postal vote and has already voted on behalf of the elector by returning their completed postal proxy vote.3