Under Police Regulations your rights in relation to breaks are defined as below.
Under 6 hours 30 minutes
Less than 7 hours 35 minutes
Less than 8 hours 40 minutes
Less than 9 hours 45 minutes
Less than 10 hours 50 minutes
10 hours 60 minutes
These should be adhered to unless there is an exigency of duty. This is not defined but can be interpreted as a pressing need or urgent requirement which cannot be avoided. This means that a break should be built into your day and the fact that it is busy or there is no one available to cover for you is foreseeable and hence should be catered for.
Breaks are also protected under the Working time Regulations which specify that if a worker's daily time is more than six hours they are entitled to a rest period. This is a minimum of 20 minutes but that should not be the norm and police regulations should be adhered to.