If it turns a IF into a RIF, then it's exactly as illegal, and has exactly the same defences.
Questions:
Do you think it's likely that you'll be found in possession of your airsoft gun(s) other than on the way to or from an airsoft skirmish?
If you are stopped under those conditions, what reasonable excuse will you offer to the offence of being in possession of an imitation firearm (realistic or otherwise makes no difference) in a public place?