Hello Andrew,
You have posed a quite tight set of conditions, a car budget of 12K for 6 seats and a caravan of MPTLM 1700Kg.
Unless you particularly need a 4x4 for other non-towing activities, then in my view an MPV will probably meet you needs best.
MPV's tend to give you the height of a 4x4 for visibility, the running costs closer to a saloon car (parts, servicing and fuel) probably more room inside than a 4x4, and certainly better rear seats and road handling.
Your budget is not likely to find you a new vehicle so you will be looking at the second hand market.
As for the weight ratio, 85% is only a guideline not a rule, so whilst it is a sensible target don't be too concerned if you cannot meet it. What is important is to check with the manufacture for the maximum permitted tow load, as not all MPV's can pull 1700.
As with all towing, be sensible, and if in doubt slow down.
Diesel is almost certainly your best bet, and look for a healthy Bhp figure. The VW/SEAT versions have range of power options. Don't dismiss the Renault Espace, especially the latest version that have a 3L diesel option, and a health gross train weight.