Under the Fraud Act 2006, if you start a rival business whilst still an employee, your employer can dismiss you for gross misconduct and make further civil claims, such as:
Breach of contract – your employer might prevent you from working for, or starting, a rival business that might cause loss to their business for a limited period.
Breach of mutual trust and confidence.
Breach of duty of good faith.
Breach of fiduciary duty (for instance, in the role of director or senior management) – you are obliged to account for all profits as a result of the breach of duty.