As Beefy says, 33bhp for 2 years, doesn't matter if you turn 21 before then. If you do a test on a 125, you are limited to 33bhp for 2 years, however old you are. If you do it on your 17th birthday then your restriction runs out on your 19th birthday.
You could have a VMAX restricted to 33bhp if you wanted/could afford to, cc is not the issue.
21 is the earliest age at which you can do accelerated or direct access which allows you to ride anything you want.
The 750cc thing your insurance company states is claptrap. Unless of course they have some kind of policy of not insuring younger riders on larger bikes. Nothing to do with the law though.
TBH I'm surprised you don't know this if you have done the 125 test.