To be pedantic - being an airsoft skirmisher is insufficient. The defence is the
intent to use the resulting RIF for airsoft skirmishing.
Under section 36.1c on the sale of RIFs an offence would be committed by the seller.
Under section 36.1b painting an IF into a RIF would be 'modification' and the offence would be committed by the painter.
Defences to these are under section 37, but airsoft skirmishing is not in the legislation. Section 37.5 allows for the Secretary of State to use 'statutory instrument' for additional defences, which is where airsoft skirmishing comes in.
https://www.legislation.gov.uk/ukpga/2006/38/section/36
https://www.legislation.gov.uk/ukpga/2006/38/section/37