I specifically asked about that when I insured my Range Rover with the NFU - they said it's covered on my policy.
Ouch! Are you sure it's just young drivers, or is it some other factor? I insure my '93 Range Rover with them - admittedly it's only worth a couple of grand (although it didn't seem to make any difference when it was insured for four), but I'm 29, I have fully comp cover for any driver over 25, 22K a year mileage and I live in a relatively high car crime area of Birmingham. About £750/year, including interest for paying monthly installments.
No problems with non-performance enhancing mods, although it did take a while to sort out permission for the Megasquirt'n'EDIS, and I've got to get an engineers report done on that once I've installed it.
Edited to say: They don't need an engineers report into the engine change though...
I've never had to claim from them, but they weren't too bad when I had to notify them of a slight mishap on the M6 last year (shed load ). I did have to correct two or three serious mistakes in the details they recorded, but they did at least deal with that properly within their normal processes.