A rest break is an uninterrupted period of at least 20 minutes, during which work should not be undertaken. You should be able to take it away from your workstation. Your employer is not allowed to make the rest break up to 20 minutes by offering several short breaks of under twenty minutes. The break should be continuous.
A period of downtime when you are allowed to stop working but must stay in contact with your employer is not a rest break, even if it turns out at the end of the break that it was uninterrupted.
Rest periods include breaks during the working day, breaks between working days, and weekly rest periods.
In addition to the specific requirement to provide rest breaks, your employer is required by the Working Time Regulations to provide adequate rest breaks where the work pattern puts workers’ health and safety at risk, in particular because work is monotonous or the work rate is predetermined.