I don't think the law specifies what form of agreement is required - however, an employer must confirm all changes in writing. Agreement at this point is implic

I don't think the law specifies what form of agreement is required - however, an employer must confirm all changes in writing. Agreement at this point is implicit - if the employee does not agree with the changes, they have the right to refuse to work, to take the case to a tribunal, or in some cases to resign and claim constructive dismissal. I know that if an employee doesn't agree with contract changes, they should take action immediately; continuing to work and not expressing your disagreement with the changes after you've been informed of them can be construes as implied agreement. Employment law in the UK seems pretty worker-friendly in the sense that most companies will take great pains to avoid the risk of employment tribunals and other employee action, but I concede it's a bit less formalised than in other jurisdictions.