Jeez, Keanu Reeves is all over the place. But if I'm reading correctly, he only holds actual citizenship in Canada. Not sure what "entitled to British citizenship" means.
As a Canadian citizen, if any of
Keanu's parents or parent's parents were/are citizens of the United Kingdom, whether through birthplace or via naturalisation,
he's eligible to become a British citizen also, if he so desires. His mother is English (from Essex). Since his father was an American from Hawaii, he's also American. As far as I know, he only holds a Canadian passport.
Further twist: he was born in Beirut. No idea of whether he is eligible for a Lebanese passport in addition to a UK and an American one.