Unlawful trespass at Knaresborough Site

Well bikes didn’t make it go up and they closed before anything waiting would have been able to nip in. However, I suppose you could put a pad for exit. Some CAMC sites require the card, some open automatically on exit.
Not cheap but I suppose ANPR coded to open for the duration of your stay would be a better option.

Entry exit cards can be Deleted User off the system

Only if they have been coded as individual entities and of course they know which one was missing. Otherwise you would have to block all. 

The site staff will have a record of those entitled  to be on site all cards normally have a unique code and of those leaving on that day whose card. would be of those that were taken ,which will be on record 

Another, rather expensive, option is to have a barrier that rises from the ground and has to be lowered by staff to let people in. 

This then leaves the itinerants to break down the exit barrier. Again, could have a spike strip across the line unless your car /motor home 's reg. number has been keyed into the system from the outset. The spikes would then safely drop into the ground.   

Similarly there could be spikes facing outwards that are levelled out when a vehicle rolls over them from the correct direction. 

Shame it is not France. I remember a few years ago the site we were on had "visitors". That afternoon word went around to campers to stay put. The armed police blocked the road, came on site, arrested the "visitors" and removed vans and cars. Quick, effective and quiet.

Colin