As you said they don't like big changes hence the 25% max otherwise they wont cover, but for cost under the 25% as a stock car sky was double the price, HIC over 3 times and greenlight wont even cover.
Youll be surprised at the difference in cost tho for younger drivers £612 for GTi, about £4000 for an evo 9 360, £9993 for a 2009 M3