No. That would not be illegal, although you could understand that obviously there would be questions asked if you walked through the scanners at Heathrow with a RIF in your bag. Specifically, the law on the matter says that it is illegal to import a RIF, unless it is for one of the following reasons:
A It is for the purposes of a museum or gallery;
B It is for the purposes of theatrical performances and of rehearsals for such performances;
C It is for the production of films (within the meaning of Part 1 of the Copyright, Designs and Patents Act 1988 (c. 48)_see section 5B of that Act);
D It is for the production of television programmes (within the meaning of the Communications Act 2003 (c. 21)_see section 405(1) of that Act);
E It is for the organisation and holding of historical re-enactments organised and held by persons specified or described for the purposes of this section by regulations made by the Secretary of State;
F It is for the purposes of functions that a person has in his capacity as a person in the service of Her Majesty.
Reason E is the one related to airsoft, and the easiest way to demonstrate that, is with a UKARA registration number, which is why people get them.