his artiste to drink alcohol at The Brits because she was recording the next day?

That would depend on the artist, in my opinion, but do tell.

Sensible manager - looking after his artist's best interests.

That opens another can of worms MM.
If the artist is used to being in, lets say, a "party atmostphere",they could well feel awkward on the night and less creative the next day as a result.
You could not have taken, for example, the Alex Harvey Band, to the Brits, knowing they were in the studio the next day and expect them to not have a tipple.
They would have just been uncomfortable.

What I meant, is that the artist's longterm career (ie the recording session the next day) is far more important than a ligging session at The Brits.

Arctic Monkeys didn't even go to The Brits this year - putting their US career first.

If you put your priorities in the right order - you will stand a better chance of winning The Brits anyway.

were they allowed to have a cigarette?

Ah! The smoking debate on singing re-emerges.
Simple fact is some singers sing better on it, some suffer as a result.
I was told that Pavarotti has a single cigarette before a performance.
This situation with the smoking will of course be great for autograph hunters when the ban comes in, just hang around outside the gig, and get rather a lot of signatures.
I`m pretty sure the Brits was non smoking, the fog in the hotel I stayed at with some of the guests later reeked of desperation for a fag, I was awoken at 4 am by a door slamming , and a guest yelling "I hate this place it stinks of smoke!"