I'm not some super terror group but if the EU started putting in border posts again i'd just get out of the car and walk across a field near the road. Unless of course you're an advocate of thousands of miles of barbed wire and fencing?
It would be fairly easy to get across Europe without a passport, the only worry would be random police checks, which are completely unrelated to immigration and free movement. The UK has a major advantage in the 20 mile wide bit of water, which is why we can sustain passport control a lot easier than other european countries. The biggest problem was the fact all got back into the EU so easily.