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I think they work 24/7 though. We went to a test centre on a Sunday morning and had the result the following Monday by 8AM. The delays must be because of the mail. A friend and his housemate had done a home test, sent it back together and the housemate got his result back a couple of days later. My friend had to wait a lot longer to get his result somehow (apparently due to an admin fault on his behalf)
48 working hours probably.