> The King is the executive and the Parliament is the legislature. Both systems have courts.

At the time Montesquieu was writing (there has been some practical evolution towards an independent judiciary since) Parliament held not only legislative power but also (in the House of Lords -- which at the time was also the dominant House in terms of the legislative function, though that is very much not true now) the judicial power as well.

Again, the then-current British system was one of the primary reference points which informed Montesquieu's model (and he clearly saw it as far superior to the French system of the time), but it wasn't designed around the three-way separation of powers with mutual checks and balances the way the US system under the Constitution was.