I dont think he is using his own insurance policy. The issue with the wide arch conversion was the insurance was through some kind of fleet policy, and the mods were too much. At least, thats as much as i have read into the situation.
Certainly would never ever get his own insurance policy on a remapped GTI at 18.
regardless if it were his insurance or not, whatever method, you consult insurance before modding a car let alone spending £4000 or whatever on a body kit only to find they'll no longer insure you! just seems like absolute madness! obv a gti at 18 is very young, let alone trying to remap it! either no insurance again, or a premium of doom!