Why should employers take into account collective consultation when varying employees' contractual terms and conditions?
For the purposes of the duty to consult collectively, the definition of "redundancy" under s.195 of the Trade Union and Labour Relations (Consolidation) Act 1992 covers dismissal for any reason not related to the individual. This includes the situation where an employer proposes to dismiss (and offer re-engagement to) employees who do not agree to a proposed change to their terms and conditions of employment (GMB v Man Truck & Bus UK Ltd  IRLR 636 EAT).
Collective consultation should take place as early as possible to give employees adequate warning of the proposed changes. As a minimum, it must begin at least 45 days before the first dismissal takes effect if the employer is proposing to dismiss 100 or more employees within a period of 90 days or less, and at least 30 days before the first dismissal if the proposed number of dismissals is between 20 and 99.
Normally, when an employer first seeks agreement to a change to the employment contract, it will not know how many employees will agree to the change, or whether or not it will need to consider dismissing and re-engaging some of them. Therefore, to avoid delay, it is prudent for employers to consult collectively from the outset, where the changes will affect 20 or more employees.