Right then, lets see if we can straighten you out
If you take your test on a borrowed 125, the licence you get allows you to ride any bike up to 33 bhp, like this
one. After 2 years you can continue to ride it, de-restrict it or sell it and buy any bike you want.
If you hit 21 before the 2 years are up you can take another test to allow you to ride any bike,
or as you say, you can wait until you are 21 and take the direct access test to ride any bike