As an ex-police officer, quick heads up (FYI please don't think I'm trying to scare you here!)
If you've got permission to use the airsoft rifle in a field go for it. However what I would suggest is calling 101 and informing the local police exactly when and where you're going to be doing it. They'll set up an incident and if any calls go into 999 about someone shooting in the field, they'll still send officers out, but they'll be aware that it's more than likely you with an airsoft gun.
Very few officers have firearm training and if you're not an airsofter yourself, you're very unlikely to have seen the hit. I'd hate for you to have armed police rolling up on you. However if that does happen, remember you've not done anything wrong. If you hear sirens/see a police car, put the gun on the floor and step away and do exactly as they say.