My wife, who is Canadian has recently received British citizenship.
We want to apply for a British passport for her but on the form it seems like she has to send her current Canadian passport in with her application. Is this right? And why is it necessary
Short answer: yes. It seems to be necessary because of a UK law or regulation that consists of matching your name on all valid passports you hold.
British passport applicants who have citizenship in other countries need to send any uncancelled passports from their other nationalities, and the name on the foreign passport needs to match the name on the certificate of registration or naturalisation.
You used to be able to send color photocopies of all pages of foreign passports. I’m not sure if that’s still acceptable.