The UK has no general statutory 'right to disconnect' that makes every after-hours email unlawful. Your contract, working arrangements and the actual demands placed on you matter. A message sent at 9pm is not by itself a breach; required work outside your agreed hours may count as working time.
Working-time rules normally limit the average week to 48 hours over a 17-week reference period, subject to exceptions and a voluntary written opt-out for adults. Workers generally have 11 consecutive hours' daily rest, 24 hours' weekly rest (or 48 hours over two weeks), and a 20-minute break when working over six hours. Exceptions and compensatory rest can apply. Time actually spent handling required work messages may count as working time; availability or on-call arrangements need separate assessment. Check all your jobs when considering total hours.
An opt-out of the 48-hour limit does not remove rest and holiday rights. You can withdraw an opt-out by giving the agreed notice, no more than three months. Check your contract or rota before announcing that you will never reply outside a particular time.
Employers must manage workplace health-and-safety risks, including work-related stress. If a long-term mental or physical condition meets the Equality Act disability definition in Great Britain, reasonable adjustments may be required; stress or burnout alone is not automatically a disability. Northern Ireland has separate disability-discrimination rules. Repeated unreasonable pressure may be relevant to a grievance or, in serious cases, a possible constructive-dismissal claim, but resignation and a successful claim do not automatically follow from out-of-hours contact. Get advice before resigning.
Keep a log of actual work time, messages and expectations. Check your contract, policy and any opt-out. You might write: 'My normal hours are [X–Y]. Is a response outside those hours required, and if so how should that time be recorded?' If the demands continue or affect your health, ask about adjustments or raise the issue with your manager or formally through a grievance. Seek independent advice if you face sanctions, unsafe hours or health effects.
Explore: https://myredundancyrights.co.uk/legality-checker · https://myredundancyrights.co.uk/my-rights · https://myredundancyrights.co.uk/action-plan
Official sources: Acas working-time rules; Acas 48-hour weekly maximum; Acas rest breaks; EHRC disability discrimination (GB). This is general information; Northern Ireland employment/equality law differs.