Replacing (physical keyb) language

Assuming you are talking about a non-retina Pro...

The US English keyboards have a slightly different key layout. Easiest way to identify is to look at the return key. On US English it is a horizontal rectangle. On most other language keyboards it is an inverted "L" shape. See images below.

I believe that the UK English keyboard is the inverted L type so you could change to one of those.

The MacBook/OS automatically detect what keyboard is attached so nothing else but changing required. It is a tedious job due to the billion tiny screws holding it on but it is not particularly difficult. I have done it dozens of times. Fixit may have a guide. You need to remove the logic board and Optical drive. Do out in an ESD protected environment.