"We're a half hour by car from Brighton." - czy forma "a half hour" jest właściwa?

Jeżeli masz na myśli "pół godziny", to prawidłowo jest "half an hour"

przykłady z Cambridge dictionary. Może pomóc, ale nie koniecznie:
The dollar surged against the yen in the final half hour of trading.
Half an hour later, she was smiling and chatting as if nothing had happened.
She is to host a new half-hour show that will be broadcast every weekday evening.
Trains for Washington depart on the/every half hour (= at 10.30, 11.30, etc.).
Once Mrs Kirkpatrick starts talking to you, you're stuck with her for a good half hour.
The train doesn't leave for another half hour yet.
Then we had to sit through another half hour of tedious questions.
The first half hour of the day is usually spent on correspondence, emails and paperwork.

"A half hour" jest poprawne ;) To prawda, większość z nas jest przyzwyczajona do "half an hour", ale niektóre źródła podają, że są miejsca na mapie, gdzie używa się "a half hour" na porządku dziennym.