The only reason why Dirty Dancing and Top Gun were 15s was because the only other option was PG, which they clearly aren't. It was that gaping void which first 12, and then 12A, filled.
In the case of The Internship, the BBC detailed classification notes appear to cover most of the OP's concerns. As it's a 12A, parents are expected to check if they have concerns. It appears to describe a rather different film to the OP's account: "hundreds of f words" versus "a single use of strong language". I guess the BBFC must have just not noticed.
"THE INTERNSHIP is a US comedy about two salesmen who lose their jobs and apply for internships at Google. It is rated 12A for moderate sex references.
There are a number of visual and verbal sex references throughout, including a scene in which a man dries his trousers under a hand dryer after apparently becoming too excited. Although some of the references are crude, they are comic in nature and generally involve innuendo or comic misunderstandings, rather than anything more detailed. In one scene, the two central characters take their fellow interns to a club in which women dance in lingerie. No nudity is shown.
There is a single use of strong language ('fk'), which is not aggressive or directed at another person. The end credits also list one song used in the film as 'F* U'. There is some moderate and mild bad language, including uses of 'wanker', 'bitch', 'pussy', 'dick', 'shit', 'screw', 'piss', 'ass', 'hell', 'crap', 'Jesus' and 'God'. There is also a middle finger gesture directed at another man.
At one point a fight breaks out, involving an exchange of blows, and there are other scenes in which men are punched in the stomachs. However, the violence is undetailed and generally comic.
Some scene show adults drinking alcohol, including shots being drunk in a nightclub. A hung over man is subsequently seen throwing up into a bin. There is a brief verbal drug reference.
No-one younger than 12 may see a 12A film in a cinema unless accompanied by an adult. No-one younger than 12 may rent or buy a 12 rated video or DVD."