Aggravated trespass is a civil offence, but armed trespass is a criminal one. If you can get film or photo evidence of any of them with a weapon then that will help. Of course, should it ever get that far the police will be virtually powerless to act as they seem (note 'seem') to be afraid to take the pikeys on.
If you happen to have a handy tractor nearby and their vehicles are parked on your land, a load of manure dumped on the vehicles might get their attention. Or you could try to find some way of blocking their vehicles in whilst you await the arrival of the blue and yellow helicopter. Just have an eye for your own safety.
Another good way to get a quick response would be to ring 999 and report a group of armed trespassers on your land, or that you had discovered burglars and YOU were holding them at gunpoint....