Check the routing of the cable from the outside. You don't say make of van, but Bailey for instance will present it near the TV aerial amp so that it can be used to fed a TV in the awning.
To use it 'the other way round' you need to identify the outside cable near the amp and the cable feeding the socket inside the van and connect them together - note NOT through the amp. If they are TV (a.k.a. Belling-Lee) connectors then you will need a simple plug-plug or socket-socket adapter: If the connectors are F-type (more common these days) you will need a 'barrel' connector.
Where are you in Zummerzet? My experience is that TV coverage is not too bad - I was involved with DSO in 2010 and 2011 in that area. I would add that quite a few CMHC sites still have pillar signal feeds from the days of analogue when a stronger signal was needed. They have not corrected these entries since which misleads as digital TV will work with weaker received signals, and due to the different way digital signals are measured, most transmitters radiate more power. Have a look at Wolfbane.com under UK TV reception. It will give you a table of serving transmitters for your location and their bearings. If the signal is shown as about 44 or higher you should be able to receive a signal on your van installation.
Come back here if you need more help.