Bogus premise is indeed bogus. The issue is VCRA 2006, not any of the ways around it that allow retailers, importers and airsoft to exist.
You didn't have to.
If you were stopped going to or from a skirmish in possession of a RIF,
or an IF, and required to provide a reasonable excuse for purposes of
Firearms Act 1968 S19 (the proof whereof lies on you) what would you say? "Because UKARA," or in your case "
Just Cos", is not sufficient as an excuse for possession in public, and neither is it necessary as a defence to modifying an IF into a RIF.
And as you've pointed out yourself, there are ways to get a retailer to sell you a RIF independently of ever having played airsoft.
Actually, I've just spotted the delicious irony there. The one retailer that I know accepts Just Cos as their defence is
https://www.airsoftworld.net
Guess who runs the UKARA scheme?