when i was 18 had the same issue, (although this was 13yrs ago) had a standard 4 door mark 4 escort with a full body kit and had to have it insured as an rs turbo, eventually found someone who insure it for a more reasonable price through an insurance broker and actually they advised if i did by an rs turbo premium would be GBP100 more only.
then had a mark 4 escort 2 door with full cosworth kit - that was insured through Privelege and the rate was only extra couple of hundred over the standard, they are worth a shot but would phone them as opposed to doing it over the internet coz if you get a friendly agent on the lone they may be able to guide yu on how to make it cheaper.