Under the Working Time Regulations, you must be given at least either an uninterrupted 24 hours off every week, or an uninterrupted 48 hours off every two weeks.
Additionally, some employers operate a scheme whereby workers can take 'time off in lieu' (often abbreviated to TOIL). This means that employees can claim back extra time that they have been required by their employer to work, by taking equivalent time off, normally within a set period after the additional work.
If your employer operates such a scheme, you will be able to find details of it in your contract or your staff handbook. There is no statutory right to TOIL. If you don't have a scheme, or you're not happy with the scheme, you should consider joining together with your colleagues (preferably via a union) and negotiating with your employer.
Even if your employer does not operate a formal TOIL scheme, it might be worth approaching your manager about the extra work you have been doing. You may be offered time off at their discretion to compensate you for all the extra work you have been doing unpaid. Don't wait too long to request this though. The longer it has been since the extra work was done, the easier it will be for the organisation to decline.
Try to keep a written record of your manager’s requests for you to work long hours, and of your requests for support and any responses. Have a look at the HSE Management Standards on Workplace Stress, which include workload and work patterns as key stress indicators.
Flexible working is a priority for trade unions, as is minimising the amount of work workers do for free. So if there's a union at your workplace, they are more likely to have negotiated a TOIL-type agreement, or appropriate pay or overtime to compensate for this.
Working excessively long hours on a regular basis – especially if unpaid – is a good indicator of organisational issues within your workplace that need addressing, such as workload, training, support, staffing and how work is organised. Where a union is recognised, these issues are usually best tackled collaboratively by unions and employers working together, as opposed to you trying to solve the problem on your own.