On the 4 clocks of Elizabeth Tower, London, why have the X (for 10) been replaced by F?

It was the preferred font of the clock's designer Augustus Pugin. As a gothic revivalist, Pugin believed the aesthetic of the shape was better suited to the gothic style than the more traditional roman X. However, the shape is an X, just one that happens to look more like an "F" shape. This font is not just found on the Elizabeth tower but on the clocks inside the Palace of Westminster as well.

https://virtualtour.parliament.uk/bigben
https://luc.devroye.org/AWNPugin-Alphabet-1844.png