You can’t say these aren’t defences when so many shops say they are. That’s only your personal opinion.
We can say they aren’t legitimate defences
Under legislation the only defences are :
Museums
theatrical performance
Film making / TV (proper filmmaking not being a YouTuber)
Crown service (For real duties, not just being a civil servant or soldier)
Note that airsoft is not one of them. It’s an additional provision by ‘statutory instrument’ and covers ‘airsoft skirmishing on insured sites’
General shooting, collecting & wall displays are not approved in the law
(You can do them with airsoft guns once in your possession but they are not defences under the VCRA)
For general shooting you could in theory have no defence required - just sell an airsoft gun with the velocity too high and sell it as an air weapon, for which no defence or licence are required (at least in England & Wales)
In which case a shooters membership isn’t needed to validate the sale, but shows the shooter to be a bit responsible
Cosplay is also not a valid defence - this is compounded by the retailer who offers it partnering with s comicon that bans entry with RIFs (just like every UK comicon / cosplay event)
Note that ‘professional cosplayers’ come under the theatrical defence
https://www.legislation.gov.uk/ukpga/2006/38/section/37
37Specific defense applying to the offence under s. 36
(1)It shall be a defence for a person charged with an offence under section 36 in respect of any conduct to show that the conduct was for the purpose only of making the imitation firearm in question available for one or more of the purposes specified in subsection (2).
(2)Those purposes are—
(a)the purposes of a museum or gallery;
(b)the purposes of theatrical performances and of rehearsals for such performances;
(c)the production of films (within the meaning of Part 1 of the Copyright, Designs and Patents Act 1988 (c. 48)_see section 5B of that Act);
(d)the production of television programmes (within the meaning of the Communications Act 2003 (c. 21)_see section 405(1) of that Act);
(e)the organisation and holding of historical re-enactments organised and held by persons specified or described for the purposes of this section by regulations made by the Secretary of State;
(f)the purposes of functions that a person has in his capacity as a person in the service of Her Majesty.