You can get portable kits from places like maplin. I've seen them priced btween £60 and £100. The cheapest option would be to get a zone1 minidish and a scaffold tube. You can knock the scaff tube into the ground as a stand for the minidish.
That would get you 28e, 19e and 13e in the majority of england, and alot of the uk. That advice may change depending on where you want to take your caravan and what you want to recieve.