Yes but the last time a monarch vetoed a law was in 1707, and that was only because parliament asked Queen Anne to veto the law.
In reality, if King Charles started vetoing then after a short constitutional crisis, parliament would basically just change the rules of the game to say that the monarch no longer has the power to veto laws.
Wouldn’t the loophole still be if one house of the parliament co-opts King, essentially simulating American democracy where you have a Republican King and Republican Party in power allowing the Republicans to veto laws and stop monarchy reform in parliament?
In reality, if King Charles started vetoing then after a short constitutional crisis, parliament would basically just change the rules of the game to say that the monarch no longer has the power to veto laws.