Hello Rob,British passports require an eTA to travel which a PR is not eligible for, unlike a US passport.
To fly back to Canada, any eTA you had before becoming a PR may be cancelled. So if this is the case, you would need a PR card or PR Travel Document obtained from London visa office in order to board an airplane back to Canada.
Alternatively you can travel to USA and cross into Canada by land, where only passport + COPR would be enough.
My son has a US passport and PR card in Canada. He needs to fly to the US soon but his PR card got stolen. He already applied for a replacement. WIll there be a problem flying back to Canada without a PR card? He has his original COPR to show at the canadian airport immigration officer. Hope to hear any advice. Thank you