Dispatch is still that day
An order placed by 3pm on Friday is dispatched on Friday. The cutoff does not move to Thursday. An order placed after 3pm on Friday is not in Friday’s dispatch.
Weekends are not dispatch days. The terms say Friday’s orders aim to be delivered by 5.30pm the following Monday.
When Monday is not a working day
A bank holiday on Friday or Monday moves the delivery to the next working day after that. The homepage also describes a 1 to 3 day courier service, depending on the items. Use the later of those if the Monday delivery is essential.
UK delivery is free on orders over £50 ex VAT, including a Friday order.