What it was "originally" doesn't really mean that it was something that by the 15th century was accepted as a kingly right, though. Laws and customs change.
not at Common Law they don't . Except by a lawful statute. That is the whole point of Common Law. English Common law has never recognised desuetude.
If you have something referring to them as the king's by right post-Magna Carta, I'd love to see it - I'm far from an expert on English laws.
Try Cunninghams History of our Taxes etc, 1761.
Even Coke!The customs are likewise said to be part of the inheritance of the Crowne at Common Law, that is, before any Act of Parliament was made concerning them. See Dyers Reports 43 {and he is quite authoritative}, Davis Reports 8, and the same is said in Davis 165 and Coke 12 Rep 33 " .
And Magna Carta itself requires that merchants were to be able to come to buy and sell "per antiquas consuetudines". John had tried to winch up the traditional rates!. The Barons objected to the new rates, not to the levying of customs at the old rate
The way Charles used it seems to be very much against the original nature of such a thing - applying outside coastal counties and outside war time. So whatever rights Charles had to it in those contexts don't necessarily apply to his extension of it.
The Judges said they did! Which is the law.
Y' see you are exactly following the Parliamentary line. Parliament claimed "ancient right" when it was to their advantage. But when the King claimed ancient right (even when the Judges backed him up), Parliament simply claimed that King and Judges were wrong ("Because we say so, right"); and that the Kings rights were obsolete (but there's were not of course".
And Victorian historians meekly followed.