I believe s.98(1)(b) of the Employment Rights Act 1996 [1] allows dismissal for "some other substantial reason" which could be argued to cover this scenario and there is some case law such as Garside and Laycock Ltd v Booth [2]
It appears that the case in question has not yet been definitively resolved, though, and the argument made would contradict almost everything many employment lawyers have been advising for a long time about the implications of the ERA in this area. Notably, that includes a lot of advice given during the previous financial crisis just over a decade ago, when similar questions were widespread.
The conventional legal wisdom seems to have been that mandating a pay cut is not allowed, because it's not a permitted deduction from wages and it's likely a breach of contract, and that firing someone just for refusing is not allowed either, because any decisions about who to let go if the business is in financial trouble are a separate matter that should be made objectively. That would mean without reference to the refusal by any given employee to accept worse terms of employment, and without conflating a position that might now be redundant with any employee currently holding that position who might also be able to work elsewhere in the business instead.
[1] http://www.legislation.gov.uk/ukpga/1996/18/section/98 [2] https://www.bto.co.uk/blog/refusing-to-accept-a-wage-cut-%E2...