The UK working Time Regulations 1998 gives most workers who work for 6 hours or more per day the right to take a break of at least twenty minutes. Young workers aged 16 and 17 have the right to a 30 minute break if they work more than 4 and half hours per day.
These rest breaks must be taken during the 6 hour period, not at the end of it. The exact timing of the breaks are up to the employer to decide. Your employer also needs to make sure that working arrangements are such that you can take your rest break. These breaks may be unpaid. However, remember that these entitlements are the minimum that the law allows, you may have better rights through your contract of employment, so make sure you understand the policy in your own workplace.
There are some circumstances that allow employers to reschedule these breaks, as long as they give 'compensatory rest' – i.e. the same length of break later in the working day. This leeway could be applied on a regular basis to a wide range of industries that require a continuous service. These include; security, hospitals, residential homes, prisons, docks, airports, press, radio, cinema, telecoms, police, fire, ambulance, coastguard, gas, water, electricity, refuse collection and disposal, research and development and agriculture.
Crucially for Christmas, your breaks can also be delayed in any industry where 'there is a foreseeable surge in activity', but your employer does have to substitute the same length of break time within the same 6 hour or more period.