If a dismissal is based on a valid reason, but as an employer, you fail to follow a proper process, then your employee may be able to claim unfair dismissal.
There are also a number of automatically unfair reasons for dismissal, including dismissals based on:
Requesting flexible working
Being pregnant or on maternity leave
Asking for family leave
Being a trade union member or representative
Requesting something to which an employee is legally entitled
Whistleblowing
Taking action over a health and safety concern
Where an employee feels that they have no choice but to resign because of unfair treatment, they may potentially be able to claim constructive dismissal.