There isn't enough cops for that and you can't argue they aren't doing it because they will be undercover and not in uniform. Also, how the hell would that even work in practice.
Downing Street 'ideas room'
"...how about if we put undercover police in pubs/clubs to act as hidden security?"
"Forces won't have enough resources for that"
"Hey, if they're 'undercover' then nobody needs to actually see them - it'll
almost be as if they aren't there - we just tell people they are there"
"Genius! Leak it to that chap at The Times"
My genuine expectation too.