https://mods.factorio.com/mod/mods-en-gb has support for multiple mods
because of how the game manages unsupported languages, you'le have to have the mod installed and edit the config.ini file to also include "en-gb" for your locale. You can find it in: application data -> roaming -> factorio -> config -> config.ini (windows)
access it on windows with windows butten+R and put:
%appdata%/factorio/config

For vanilla games you could try opening the mod zip and navigating to the folder "locale" and instead pop that into the game folder "core"

More info:
precerser but not a derivitive of https://mods.factorio.com/mod/BritishEnglishLocale
I did use the thumbnail.png
with file editing you may currupt your game, I do not take responsibility
if you know, can you tell me how to add it as a selectable in the language gui

Planned:
Compatability Planned for:
angels (think theres some words that can be translated)
bobs (think theres some words that can be translated)

How to add it as a selector (need a file),(require mod enabled to use en-gb):
Go onto steam, on the left with the game list right click on factorio -> properties -> local files -> browse (click "browse")
Go through this process: data -> core -> locale ->(create folder called "en-gb") -> paste this file into the folder "en-gb":
https://drive.google.com/file/d/1_OfsHPQMZHmEvJmHGShQJFzcoWpzUYGk/view?usp=sharing