Caravanning over the xmas period

We fancied spending five nights (27th Dec to 1st Jan) in London with the van and wanted to book either Abbey Wood or Cristal Palace but they are fully booked for at least some of the days which I suppose is what happens when you leave things till the last minute.
Does anyone know of another good site within fairly close distance to London, or, any other nice sites near a big town/city that would be nice over that period?