What Makes a Successful constitution?Watch
If the former, then the consent of the political actors to play by its rules is the key, I would say. No constitution no matter how 'entrenched' is worth the paper it's written on if a major political actor fails to uphold their responsibilities within it.
also, the maximum amount of basic liberties in so far as they do not make parliaments useless
Both Britain and the US fail to meet that nice balance.