The £25 per day I think is pretty normal while in transit. I seem to remember paying something like that when I collected my kit.
Most kit car insurance policies don't seem to worry about engine mods - after all if you have spent 12months plus building the thing (plus several ££££££s) it's pretty unlikey you are going to wrap it arround the nearest lampost.
I think the problem you are gonna have is your younger brother - some policies you have to be 25+ (no problem for me
there's GOT to be some advantages in being an old git...)
Best advice - shop arround - there are loads of companies offering kit car insurance nowadays. When I insured my first kit car 24 years ago there was either Adrian Flux or Adrian Flux or Adrian Flux...