This depends on the terms and conditions of the employment contract. Subject to laws that set out basic minimum statutory rights, such as to the National Minimum Wage, or Statutory Sick Pay, whether or not your extra benefits are discretionary depends on the terms of your employment contract.
An employer would need to use very clear words in the contract in order for important benefits to be discretionary (i.e. entirely in the power of the employer to give, change or withdraw).
- the employer must not breach equality laws. This might happen if, for example, the employer operates a discretionary benefits system that results in less advantageous allocation of benefits for those employees who have a protected characteristic, such as disability, race, religion or gender, where differences in treatment cannot be justified;
- when making decisions about discretionary benefits, the employer must not disadvantage part-time or fixed-term employees, when compared with their full-time or permanent colleagues, unless the different treatment can be justified;
- the employer must take care not to breach the Agency Workers Regulations. Temporary agency workers are entitled to equal treatment when it comes to accessing many collective benefits;
- the employer must not withdraw benefits, even if "discretionary", to retaliate against an employee who has exercised a statutory right, for example, asking to take rest breaks under the Working Time Directive, or asking to work flexibly. This would be unlawful. There are lots of similar examples.
Even if the contract terms state clearly that a benefit is discretionary, the basis on which the benefit is accessed should not be changed without agreement. Otherwise, the employer is likely to breach the contract of employment.
Even if a benefit is clearly discretionary, an employer must not breach important implied contract terms when deciding how to exercise their discretion. The most obvious are the duties of trust and confidence and good faith. Finally, there is case law that suggests that an employer should not exercise their discretion irrationally.
The provision of discretionary benefits over a long period of time eventually crystallise into an implied term of the contract through custom and practice. It would be a breach of contract to withdraw a benefit that had crystallised into a binding legal promise. However, in practice, this kind of claim is extremely difficult to pursue.
Many good employers prefer to avoid offering purely discretionary benefits, because they carry a high risk of unlawful discrimination, individual line manager bias and favouritism. In general, you are less likely to find purely discretionary benefits in a unionised workplace.