Obviously an employer absolutely can't require employees to vote for any particular candidate (or even to vote for any candidate rather than spoiling their ballot), but if it was included as a clause in employees' contracts that they must (in company time) attend the appropriate polling station and "participate" in some defined set of elections (eg "all local and national governmental elections and referenda"), and that failing to do so was a disciplinary offence, would such a clause be enforceable? Would an employer legitimately be able to fire someone for failing to comply?
Looking for answers in relation to the UK.