You are entitled to a break of at least 20 minutes after working a maximum shift of 6 hours. That is the law. However this break does not have to be paid, and for most employees it is not paid. It consists the lunch/shift break.
Apart from that, local arrangements apply. It would be a stupid employer who did not allow an employee who moves around as part of the daily shift (a cleaner) not to be able to have a bottle of water with him/her to have during this six hour period between unpaid breaks. 'Going to get a cup of coffee' technically is not within the scope of this - it depends how far you are going to have to go to get it. Its the similar arrangement as time off to go to the loo - everyone needs to do it, but it needs to be managed if staff seem to be abusing the obvious need.
Most employers make sensible local rules - some managers don't and undoubtedly loose staff goodwill in the process.