Protesting on the highways, preventing access to others etc etc is peaceful protest. These people caused criminal damage by cutting a perimeter fence. I suspect given their age and behaviour they were tweeting or using social media - not that difficult to imagine others with far more malevolent learning of the plans and following them through the fence to commit terrorist acts.
If they wanted to protest against UK law they could have protested outside the terminal land side. They decided to go further. They are not stupid. They deserve what they got, and indeed should be relieved not to be jailed.
The problem is the message this sends out to others, and the effect it will have on airlines when next asked to deport these illegals. Civil action would help as a deterrent even if it merely resulted in bankruptcy, but I suspect it wont happen