It absolutely is illegal to be in an empty building without the permission of the owner, even if it is unoccupied, not to mention the fact that it could very well be dangerous to be there too. In fact it is both a criminal offence
and a civil offence. Not only that, since it is a criminal offence, you would not be able to provide insurance for anyone taking part in such an activity where they were potentially at risk, so it would also be illegal for you to take money for it. Being uninsured and illegal, it would also be outside the stated acceptable defences of the VCR Act, which means that in addition to being open to prosecution under that Act, the CPS would probably throw some firearms offences at you, and you'd also be open to numerous potential civil actions as well. Not forgetting that you'd be in breach of the Criminal Justice and Public Order Act, which was specifically put together to counter 'raves' and other such organised gatherings in disused empty buildings, where incidentally, that Act specifically mentions that it is illegal to throw things, which makes half of what goes on in airsoft games a non starter as well, even if everything else wasn't problematic. Last but not least, I bet you Facebok would push for you to be prosecuted if you used them to promo such an activity, they love some free publicity.
As if all that wasn't bad enough, even if you got away with nobody being injured, for which you, as the organiser, would be held responsible, it would be absolutely appallingly bad publicity for airsoft, and that's the last thing we need when the Government would love a reason to put a stop to people having airsoft replicas. We should be going out of our way to appear responsible, not seeing how many laws we can break in one day