Credits are applied before debits (for transactions on the same day), so this shouldn’t be an issue for you.
I also don’t think that bank holidays are an issue where payments are concerned - in my experience, if my pay date falls on a bank holiday I get the funds on the previous working day and debits are always on the next working day, so there should never be an issue with payments coming out before direct credits.
Someone else who is more knowledgable about BACS can confirm though.