The official answer to this question is of course “yes” – there is no law regarding exactly where one can play Airsoft. You can even play it in your back garden if you’re stupid enough. However, due to the fact that your back garden, nor your friend’s large-ish parcel of land in the woods more than likely isn’t insured for third party liability, we do not recommend actively skirmishing on uninsured private property.
However, there are laws which criminalise the action of bringing an air weapon or imitation firearm onto land that you’re not supposed to be on – this is called trespass. This variant of trespass is oddly enough one of the few forms of trespass that is criminal – and could result in a criminal record.
There is legislation on the books that prohibits projectiles from air weaponry from leaving the boundary of the land you have permission to shoot on, however airsoft replicas are not air weaponry if they fall into the Airsoft exemption. Notwithstanding the appearance of this section not having been enacted, its enactment appears in a commencement order.