Drinking up time as a legal definition disappeared in 2003, from Wikipedia:
The consumption of alcohol itself is not considered a "licensable activity" under the new Licensing Act. Therefore, "drinking-up time" (DUT) has no legal meaning and has disappeared. For many years ten minutes (and later extended to twenty minutes) was the legal dispensation which allowed the consumption of alcohol to continue after the official closing time, which in recent times meant that customers could still drink what they had already bought until 23:20, subject to the licensee's discretion. After that time consumption had to also stop.
With the end of standard permitted hours, this concession became irrelevant and there is no mention of DUT in the 2003 Act. Instead, applicants for premises licences can specify the maximum period (their "Opening Hours") for which they wish to allow their customers to stay after the time at which the sale of alcohol ends ("the terminal hour") within their Operating Schedule. Some licences do not specify opening hours at all, which allows an unspecified drinking up time, determined only by the licensee's discretion. In contrast, some licensees call for "last orders" twenty minutes (or more) before the end of the opening hours specified on their premises license