Benny, John and Lutz are correct, the 7% is just an old fashioned guidline that may have had some relavence when most vans were below 1 ton, but with average MPTLM now in the region of 1400kg it is imparactical as you have found.
In any event my observation of caravanners over the last 20 years or so is that they are not that paranoid. Most of them, including myself, try to get it to maximum whilst still staying within the cars nose weight limit. We just load the van more or less the same way after that - I have never seen anybody using a nose weight gauge prior to hitching up prior to leaving a site but I am sure there are those that do.
Are you sure tha CC are saying the insurance is invalid if the nosewight isn't 7%? What if its 5% or 10%? My copy of the CC handbook says "It is recommended that the nose weight should be varied to find the optimum for towing dependant on the actual laden weight of the caravan.Experience and research has shown that the noseweight should be approximately 7% of the actual weight for optimum stability. However,this may be resticted by the towing vehicle manufacturer's limit and the caravan's hitch limit". Nowhere do they call the 7% a "rule" a "guidline" or "the law".