If you say so.
It's a toy gun, if no one sees you shouldn't even need permission - i speak only of firing it, not playing airsoft (or brandishing).
This [both] depends upon what you are shooting, and where you are shooting it. Among you and your friends, do whatever you like. At airsoft game sites, its generally the same across the country.
Reports indicate the largest problem you will encounter will be getting it back into England, despite being entitled to.