Legally, they do NOT have to be a site member or have UKARA.
UKARA is the only GUARANTEED form of entitlement to the Skirmishers defense, in so much as the Home Office have promised us a court would accept it, however, they have not specified a list of anything else they will or will not take.
I have purchased before based on an amalgamation of the below:
-Dedicated member of many airsoft forums for a long period, showing good contribution to the communities
-Dedicated expertise of airsoft weaponry, internal and external modification, beyond the expected knowledge of a non-player
-Consulted with the HO in the formulation of the two-tone system and under-eighteen access to the sport
-Have worked for an airsoft site
-Have volunteered for an airsoft site
-Run an airsoft society for my university
-Have contributed articles and reviews to an airsoft e-zine
-Have a written record of every skirmish I have ever attended, with photographs and other information etc
-Have two testimonials, one from a firearms officer that I am a skirmisher
-UKARA/Site membership
-Have written several legal guides surrounding various aspects of airsoft and its relationship with the law
You don't need UKARA as long as you can prove beyond reasonable doubt that you are a skirmisher. How you choose to do so is between you and the other party, UKARA, it happens is just the simplest as it is totally guaranteed.