Found it fun reading all your replies to each other I just want to pop in and say I actually ran an Airsoft site I started up with some friends
The way you would need to do this is to treat it as a business and just launch it you wouldn't need to worry about paying loads of tax if you don't make profit so if you could show that all the money collected on a "gameday" was used to cover your businesses bills and non was taken for profit you would be fine
In terms of insurances its alot easier to find a woodland and get it risk assessed and maintained than it is with an indoor site or a site with more buildings, you would also need to write out waivers nullifying yourself from any blame of injury and explain that a safety brief was given at the start of the day to highlight these risks.
you can insure yourself under outdoor activities/recreational or something kind of sports insurance
With pyros id recommend going for only impact grenades and no actual pyrotechnics that make a flame.
I understand this post is very old but id be interested to hear back from you to see if you did ever find a way to make this work