The aim in the terms
The terms say internet orders received by 3pm aim to be delivered by 5.30pm the next working day. Monday to Thursday, that next working day is the following day. An order placed after 3pm is not in that day’s dispatch, so the aim moves with it.
The homepage describes the courier as a 1 to 3 day service, depending on the items in the order. Both statements are published. If the delivery has to be on a particular day, plan for the later of the two.
Friday and a bank holiday
Friday’s orders aim for the following Monday. A bank holiday on Friday or Monday moves the delivery to the next working day after that. Weekends are not dispatch days.
The cutoff does not change because the order is small. UK delivery is free over £50 ex VAT, and an order under that figure still leaves on the same dispatch rule.