What you're allowed and what you drink are really two different answers. The biggest problem is not how much but when - usually Friday and Saturday evenings, and that can lead to binge drinking which appears to be the nemesis of British culture and what the establishment is uptight about. I reckon that even if bars were allowed to open 24 hours a day binge drinking would still happen. The best answer is what you're comfortable with, always assuming your other half doesn't object to strongly