Short answer- same as the US, don’t send them until you have an offer you want to accept.
Remember that you are applying to 5 places, and you will know what offers you have by April- but then you have to narrow that to 1 or possibly 2 before you have your AP results. If the uni you want to “Firm” has given you a conditional offer you are allowed to “Insure” a 2nd.
There are 3 possible responses: you get rejected, get an offer with conditions, or get an offer with no conditions. Given that you have Calc BC (a key class) scheduled for this year, it is unlikely that you will get an unconditional offer. The conditional offer language typically goes along the lines of “offer you a place for PPE for Sept/Oct 2022 conditional on achieving a score of X on exam 1, Y on exam 2, and Z on exam 3 etc.” or “verification of the following scores” or “scores of 5 on 3 of the following exams” (nb, I have only seen Oxford do that). The list can include exams you have already taken, in which case it’s a roundabout way of saying send your official score reports for those exams.