You need to go to a wifi hotspot. It is illegal to connect to somebody's unsecured network without their permission, although as a one off you are unlikely to be caught.
However, you are unlikely to be able to connect in your caravan - unless the site has wifi and you are close to the router.
Wifi hotspots include McDonalds, Starbucks, some hotels and pubs, service stations, some public libraries...
Sometimes they are free; often you have to pay through BT OpenZone or The Cloud - both have pay as you go.
Try this to find hotspots close to you:
http://www.myhotspots.co.uk/