Weird laws that almost every Brit has (probably) broken . . .

Being Drunk in the Pub
Though lots of Brits may not be aware, it is actually an offence to be drunk while in a pub. The law is listed under section 12 of the 1872 Licensing Act, which says if any person is found to be drunk on a licensed property, they could be fined £200. The law has also stated since 2003 that it is illegal for bar staff to serve drunk people.
Ref: https://www.bpp.com/courses/law