As long as they comply with legislation such as the National Minimum Wage Act, the Working Time Regulations and the equal pay provisions of the Equality Act 2010, the terms of a contract of employment are for the employer and employees to agree.
Obviously at the start of a contract an employer is generally in a much stronger negotiating position. That is one of the reasons terms negotiated by a union representing a significant proportion of the workforce will always be better than those simply agreed to by one individual.
Employers must consult staff before they attempt to fundamentally vary a contract, for example by introducing a new shift pattern. However, if an additional benefit, such as a bonus, is genuinely paid at the discretion of (rather than automatically by) the employer, then variation or removal by the employer without consultation would not necessarily be a breach of contract.
If a benefit is provided continually over a long period, then an implied term may be created through custom and practice even if it is not spelled out in the contract. Removal or variation by the employer might therefore be a breach of contract. Such a claim may be difficult to successfully establish but could be useful for negotiating purposes.
