Incidentally, I think that the W10, P13 and several of the U routes
in
Uxbridge have had the same fare structure as every other bus in
London since
introduction.

Yes I was going to add to my original response that the W6 that was
mentioned by the OP is one I've only been aware of in recent years and
I wasn't sure if it had any history before it served Edmonton Green.

Although I believe my first response was correct as far as it went it
cannot be the whole story. I suspect that letter prefixed route
numbers were also used for the first 'midibus' routes.

This may trigger an example of a letter prefixed route that was
neither introduced as a flat fare route or as a midibus but it will do
until someone who actually knows the facts can contribute!