I’ve not personally had a BACs credit to my account yet, though I believe CeX will be using that in the coming days So I will likely be able to see how long it takes!
But according to this site it’s 3 working days:
Disadvantages of Bacs Direct Credit
Not suitable for same-day payments (takes 3 working days).
4pm is also around the deadline for some banks (if I understand correctly) for a bacs payment to be made that working day, after that it goes in next days batch