Can employees refuse to accept a variation to their terms and conditions of employment but continue to work under the new terms?
Yes, an employee can refuse to accept a variation to their contract and "work under protest". This means that the employee continues to work under the new terms, but makes it clear that they do not accept the terms. While remaining in employment, the employee may choose to bring legal action for breach of contract and/or an unlawful deduction of wages claim (if the variation involves a reduction in pay).
The employee is entitled to a reasonable period of time in which to determine whether or not to accept the new terms, even if the contractual variation has immediate practical effect. The employee can also make clear that they wish to have a trial period in order to determine whether or not to accept the new terms (Turvey and others v C W Cheyney & Son Ltd  IRLR 105 EAT). If the employee states that they are giving the new terms a trial, this does not amount to an acceptance, but if they carry on working under the new terms for longer than a reasonable trial period without raising any further objections, they may be taken to have impliedly agreed to a variation of their contract.