does this now mean *strictly speaking* to make the sale would be illegal
Strictly speaking - the
best kind of speaking - it's illegal with or without a UKARA.
A person is guilty of an offence if he sells a realistic imitation firearm.
If charged, it shall be a defence to
show that the conduct was for the purpose only of making the imitation firearm in question available for [a variety of reasons, one of which is]
the organisation and holding of permitted activities, which (probably) refers to
airsoft skirmishing.
The reason that I'm linking directly to the Act, the Regulations and the explanatory notes is that - strictly speaking - there's no unambiguous answer to your question, since there's no case law on this that we're aware of.
For my own part, I'll note that UKARA only provides some indication of what a buyer
has done, not what they
will do, and it's a curious defence which requires the seller to predict the future.
A player could do 3 days in 2 months, then go 365 days without playing again, and still be assumed (by UKARA) to be an active skirmisher. That's a bit of a stretch to adducing a defence "for the purposes only of" airsoft skirmishing.
But the tl;dr version is anyone who's managed to ever get a UKARA is unlikely to be a person who'll do something with a RIF that will bring it to the attention of the State. That means that in practice it's just not going to be an issue for you.