In general, the answer must be 'No'.
However it's not uncommon for patrols to be out on the road for many hours, so senior officers are unlikely to object to their junior colleagues calling into a café to grab a coffee, or to nipping into a supermarket for a sandwich, etc. Indeed many smaller shops (particularly corner shops selling alcohol) are happy to provide police officers with freebies, as they feel reassured by having a regular police presence on the premises.
Chris
PS: I've just read through that last sentence again. By 'freebies' I meant sandwiches or cans of Coke, etc. I wasn't implying that the coppers are given free booze(!) - only that off-licences are particularly vulnerable to shop-lifting and robberies!