Can I replace a "UK" keyboard layout` Macbook Pro 2016 top case with a "US" top case?

Is it possible to replace my UK layout top case with a new US top case?

Also is it possible to replace this 2016 model top case with a 2018 top case?

Whilst the topcase-and-keyboard is interchangable (within years - you can't put the 2016 guts into a 2018 body), changing the keyboard's nationality layout does not change what the machine knows as its native tongue. Yes, it is possible to change the keyboard layout in the OS to match the new languagr-layout, but that only takes effect once you have logged in - at the login screen, the keyboard will still be using the original language layout.