You only have a clear right to a bonus if it is specified in your contract of employment. In such situations, it may only apply if you achieve specified targets, e.g. sell a certain number of products or increase your productivity by a certain amount.
However, if it has become 'custom and practice' to pay one – in other words, if you have automatically had one every year so far, and this is well-known – you may be able to argue that it has become an unwritten part of your contract, and claim that an unlawful deduction has been made from your wages.
When deciding whether or not to award a bonus, the law does not always demand that employers act “reasonably”, but they should not act irrationally. If you have been denied a bonus whereas your colleagues have not, and you cannot see any obvious rational explanation for treating you differently (e.g. something about your work performance compared with theirs), this might be an indication that there is something unlawful going on.
For example, it would be unlawful to deny you a bonus to punish you for exercising a statutory right such as your right to join a trade union, or because you have a protected characteristic – for example, because your manager is homophobic and you are gay.