IMPORTANT! If you still want achievements on steam please install through the method that doesn't require running the mod

Important Summary (ish)

This mod doesn't overwrite the existing en locale and instead uses a different unsupported locale en-gb
Mod support is on a separate mod called mods-en-gb
Includes Space Age, Quality and Elevated Rails DLC support inside as of version mod 0.2.0

How to Install (because of how the game manages unsupported languages)

via editing config.ini but when you switch language once you'll have to do again

Find config.ini in User data directory
Head to section [general] and put the setting as: locale=en-gb as seen in the image

via adding a dedicated selector for en-gb

Find mods folder in User data directory
open the .zip (mod itself) and navigate to a folder called "basegame" which this folder is next to other files including info.json
Navigate down it through basegame -> core -> locale -> en-gb -> info.json & copy this file
Find your Factorio Application Directory
navigate down data -> core -> locale & make a folder en-gb, in this folder paste the file you copied
when you boot up the game it should now be select-able, this should not override unless you change your game install location

via complete install (with this you can actually run en-gb whilst not running the mod)

Find mods folder in User data directory
open the .zip (mod itself) and navigate to a folder called "basegame" which this folder is next to other files including info.json
Copy base & core from the basegame folder (If you have the DLC copy over elevated-rails , quality and space-age as well)
Find your Factorio Application Directory
go into the data folder
paste
This should run, whenever this mod updates though I would recommend reinstalling to install en-gb updates if you use this way

More info:

Inspired but not a derivative of https://mods.factorio.com/mod/BritishEnglishLocale but the thumbnail.png is from there
with file editing you may corrupt your game even though I wouldnt want it to currupt on your side, I do not take responsibility
if you know, can you tell me how to add it as a selectable in the language gui via just the mod