BCAP Code rules 3.25-3.27 apply. CAP also have a help note, that will be helpful.
Advertisements must not describe a product or service as “free” or similar if the consumer has to pay for anything other than the unavoidable cost of responding to the promotion and collecting or paying for delivery of the item.
If the service being promoted as ‘free’ is normally provided at no charge, Radiocentre takes the view that to promote such a service as ‘free’ is misleading. For example, an advertisement for a newspaper issued with a CD as a one-off gift may describe the CD as ‘free’. However, an advertisement for a newspaper offering a regular supplement (e.g. a classified recruitment section) may not describe the supplement as “free”. Instead, the supplement could be described as ‘inclusive’.
If an element of a product is indivisible from the product as a whole, then it cannot be described as “free”. An online retailer cannot describe its delivery as “free” if there is no other way of receiving the goods.
If a retailer is online only, then it cannot describe its delivery as “free” – the delivery is an indivisible aspect of purchasing the product. However, where an online retailer also has a bricks and mortar store, and the online and in-store prices are the same, the online delivery can be described as free.
Helpnote – Example Uses of Free