Is that right though? Presumably one can enter Canada on any passport one legally holds, provided the other paperwork (visas, for example, if needed) are in order.

I think it more likely that questions will be raised because, presumably, the personal details (name, date of birth, place of birth) will tally with the holder's criminal record – if that record is available to the Canadian authorities at all (which it may well be). This won't prove anything, necessarily, but it may arouse suspicions.

I think it's unlikely that the Canadian authorities will be particularly concerned, prima facie, with other countries' stamps or lack of them in a British passport.