Canada will not revoke your PR card unless you lied on your application. My guess is that UK immigration kept a copy to justify why they gave you a 24 hr. transit visa in case you used that visa to disappear in the UK.
Thank you Leaon,
i do not think any one who got Canadian PR wuld like to disappear in UK. NO WAY, I lived there for 8 years, not again, had enough.