The police have crossed the line.

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If he did drink wouldn't that be...illegal?

I'm sure that nobody here knowingly breaks any laws in real life!

Not sure how it works in the UK mate, but here as a teen it is legal to drink.
It is illegal to sell alcohol for teens.

BBC News says:

UNDER FIVES
It is illegal to give an alcoholic drink to a child under five except under medical supervision in an emergency.

It is thought the Children Act 1908 made provision for parents to give alcohol to a very young child, where it could be considered an anaesthetic in cases of acute injury.

UNDER 16s
Children under 16 can go anywhere in a pub as long as they are supervised by an adult, but cannot have any alcoholic drinks.

However some premises may be subject to licensing conditions preventing them from entering, for example pubs which have experienced problems with underage drinking.

16 TO 17-YEAR-OLDS
Young people aged 16 or 17 can drink beer, wine or cider with a meal if it is bought by an adult and they are accompanied by an adult.

It is illegal for this age group to drink spirits in pubs even with a meal.

UNDER 18s
It is illegal for people under 18 to buy alcohol in a pub, off-licence, shop or elsewhere.

And in most cases, it is against the law for anyone to buy alcohol for someone under 18 to drink in a pub or a public place.

It is legal for adults to buy alcohol for children over four to drink in the home.

Under the Confiscation of Alcohol (Young Persons) Act 1997, police have powers to confiscate alcohol from under 18s drinking in public places including streets and parks.