My employer made my job unbearable until I quit, is that constructive dismissal?

Constructive dismissal is a claim you can bring when you resign in response to your employer's serious breach of your employment contract. It is governed by the Employment Rights Act 1996 and established case law.

For a claim to succeed, you must show: (1) your employer committed a fundamental (repudiatory) breach of your contract; (2) you resigned in response to that breach and not for another reason; and (3) you did not affirm the breach by accepting it (by continuing to work for a prolonged period after the breach without protest).

The breach is usually of the implied duty of trust and confidence — the obligation not to behave in a manner calculated to destroy the employment relationship. This can cover: serious bullying or harassment; unjustified disciplinary action; significant pay cuts without agreement; failing to investigate grievances; imposing unreasonable working conditions; or subjecting you to a hostile work environment.

A single serious act can be sufficient. Alternatively, a series of acts, the last of which is the "last straw," can cumulatively breach the implied term.

You need two years of continuous employment to bring an unfair dismissal claim (of which constructive dismissal is one type), unless the dismissal relates to a protected characteristic or whistleblowing, where day-one protection applies.

Raise a formal grievance before resigning if possible. This protects your position and shows you gave the employer an opportunity to remedy the situation. Resignation should be as a last resort and clearly stated as being in response to the employer's breach.