I am planning on going to Canada on a working holiday visa. I know you can renew UK passports with 9 months still valid on your passport and they add this to the new one, but can I apply for a new one now 11 months before it expires and they add just 9 months on or do I have to wait until October before I can renew as this would exactly 9 months before it expires?
In terms of getting a new passport, there's no problem ever renewing a passport early if you need it.
You can renew or replace your passport at any time. It costs £72.50 by post or £81.25 through the Post Office’s Passport Check & Send service. Time left on an existing passport is added to your new one - up to a maximum of 9 months.