First of all, UKARA is not a license. It's a defence for an exemption.
Secondly even if you had a FAC, you would still be unable to buy RIFs.
I assume as you're asking here, your purchase has been pulled by customs and they've asked you for your UKARA? If so, sorry but that's getting confiscated and destroyed. I hope it didn't cost too much.
I suggest if you want to own RIFs, just go and play Airsoft. It's really not a big ask.