As a customer, it's my liberty and me that risks the court case/criminal record/fine...
I'm not convinced that's right Martin - having read section 141 it states (my emphasis):
http://www.opsi.gov.uk/acts/acts1988/ukpga_19880033_en_14 said:Offensive weapons .
(1) Any person who manufactures, sells or hires or offers for sale or hire, exposes or has in his possession for the purpose of sale or hire, or lends or gives to any other person, a weapon to which this section applies shall be guilty of an offence and liable on summary conviction to imprisonment for a term not exceeding six months or to a fine not exceeding level 5 on the standard scale or both. .
(2) The Secretary of State may by order made by statutory instrument direct that this section shall apply to any description of weapon specified in the order except .
(a) any weapon subject to the [1968 c. 27.] Firearms Act 1968; and .
(b) crossbows.
(3) A statutory instrument containing an order under this section shall not be made unless a draft of the instrument has been laid before Parliament and has been approved by a resolution of each House of Parliament.
(4) The importation of a weapon to which this section applies is hereby prohibited.
There's no mention of possession, ownership or use only the intent to supply to another person a weapon proscribed by the act. If you buy the weapon in good faith in the UK (important that as importation will be down to the purchaser not the vendor in the case of personal importation) is it not so that you have a defence if charged under section 141? As you rightly say, it will take a court case to clarify it further, but as I read it, the purchaser is not considered by section 141 unless purchasing on behalf of someone else.
Cheers,