breaking buggies down is so disheartining. i got a vmax that doesnt fit in my car (im searching for a knackered rusty pickup of some sorts) taking it to bits scratches the finish, takes ages & is all round not great! I rekon you could make something to clip it to your seatpost. I dont think the flexi buggies break down into bits that are small enough to fit in a bag all that well. As for buggying on common land i dunno, but you deffo should get some sort of insurance. There are tons of threads on here about insurance but the bbc insurance covers you for pretty much anywhere and its nice & cheep!
Good luck with the buggying mate, you'll love it
fettler