In airsofting in the uk, Are you allowed to lend your friend an airsoft gun to skirmish with?

I assume you are tlaking about Realistic Imitation Firearms (RIF’s) rather than Two tones?

Yes. You can even give (no money, goods or services must be traded in return) a friend a RIF as long as they are over 18. The Violent Crimes Reduction Act (VCRA) only covers the selling, importing or manufacturing of RIF’s not ownership.