My understanding is that teams are used in the plural in BrE and in the singular in AmE.

Can any native British and American make this clear, please? Do Americans always use the singular at all?

Hello Anon

In BrE, it would be more usual to say:

1. Manchester United are running out onto the pitch.

2. Manchester United is one of the great English football clubs.

In #1, Man. Utd is a group of people: hence the plural verb. (If you say "is running", it conjures up the image of a many-legged conga.)

In #2, Man. Utd is an entity: hence the singular verb.

Some speakers of BrE insist on a singular verb in all contexts, though.

(With luck, some AmE speakers will pass by shortly.)

Americans almost always use the singular verb if the surface structure of the subject is singular. Americans make little use of the idea of "notional subject".

The whole family is gathering at Aunt Em's for Christmas dinner.
The committee is ready to announce the results.
The team has won all but one of its games this year.

But note:
Proctor and Gamble is a big company.