Here's the train of thought I'm inviting you to board.
Modifying an IF into a RIF is an offence. But in practice that's only an issue if it comes to the attention of a copper.
And that's most likely to happen when you're found to be in possession of the imitation firearm in a public place, yes?
If that happens, your immediate problem is not VCRA, it's Firearms Act 1968 S19, to which you need to produce and prove a reasonable excuse for possession at that time. Having a UKARA number is neither here nor there with regards to possession. And it's immaterial whether it's an IF or a RIF - we know of a conviction for possession of a bright orange springer Glock.
So: your reasonable excuse is that you're on your way to or from an airsoft skirmish at an insured site, right? This is why I recommend that everyone keeps booking information or some sort of attendance confirmation to hand while in transit, even if it's just an "I'm attending" on a Facebook event.
OK, so you've cleared that hurdle and satisfied the constable that you have a reasonable excuse to possess an IF or a RIF at that time. Then even in the very unlikely event that they also know that it's an offence to convert an IF into a RIF, and even if they care, and even if they suspect you of doing so, and even if they make the accusation, and even if you're daft enough to admit it, then you can still produce a defence to that, which is that it's been done for the purposes of a permitted activity, i.e. ... an airsoft skirmish at an insured site.
I'm not advocating any course of action, just pointing out that if you can produce a suitable answer to "Why do you have that IF?" (and you'd best be prepared to do so), then the same defence very likely applies to "Why did you turn that IF into a RIF?"