Its legal if they have a full UKARA, but if there is
any exchange of goods or money (even leaving money lying around for them to take, internet or whatever) it can be used to accuse the 'Seller' and potentially you of illegal activity.
Technically ownership is not illegal for example if you had the gun from before the VCRA law. I would ring or go to the site you want to skirmish at before and ask because you can argue with the marshalls all you want that its technically legal but at the end of the day its their site and its their rules with regards to insurance and law. The one site I've been at doesn't ask about UKARA if you bring a RIF and only asks if you want them to put you on the list for registering
but this may differ between sites.
I would make sure you can contact whoever gifts you the gun though because if the police stop you and ask about it they can and probably will confiscate your gun until their satisfied that its legal and that would ruin your skirmish.
http://www.airsoft-forums.co.uk/?showtopic=9772