OK.
So primarily, you need to have a defence to convince the retailer to sell to you.
Secondly, You may need some proof of your involvement with airsofting to convince a police officer that you have a reasonable excuse to carry in public.
On the first, it depends what the retailers collectively demand as reasonable. Up to now, its been a UKARA registration number. Unless the majority of retailers accept something else, then that is where it will stay. In many ways, any change or alternative registration method will have to be adopted by them for it to work.
Secondly, it might be obvious to the police if you are all cammed up and 200 yards from the entrance to a skirmish site, but if not, how do you easily prove this ? If you are walking down the street in street clothes with your RIF in a shoulder bag, how do you convince them ?
Site membership card with photo ID, signed log book of where you have played in the past ?