Just renewed online and sent off my old passport yesterday. The automated website informs you that it could take up to 11 weeks to process. They asked for about £105 including ‘courier fees’ for a 35 page passport, more for a 90 page, though it did not mention the number of years as I recall. (?)
I hope they return the old one, suitably cancelled of course, with my current visa stamps, and I hope the mail is reliable and nothing gets purloined in the process.
( Edit, oops, just registered nightstop’s last observation.)