Stepping back a bit, one key thing to understand is the difference between EN-GB and EN-US.
Realistically, spelling differences are minor and not difficult to understand for either set of users - here's a (non-exhaustive) set of examples:
Prefixes and Suffixes
Change to -ise/-yse. (e.g. synchronise, analyse)
Change to -our. (e.g. colour, neighbour)
Change to -re. (e.g. centre, metre)
Change to -gue. (e.g. Catalogue, dialogue)
Change to -ll-. (e.g. enrollment, modelling)
Change to -t. (e.g. leapt, learnt) (This is not always the rule!)
Depending on the context of your website, there may be other bits of copy you use that are have EN-US roots - but typically most English speakers will be able to understand them, but your experience will vary depending on the type of website it is and how niche the topics and journeys are.
Two keys things to evaluate before spending time and effort on this are:
- what is the proportion of non-US English speakers using the website currently?
- what is the business value of adding in translations for those users?
Unless you can point to anything specific, this requirement feels like a 'nice to have', and you may find your effort is spent better on other languages or features in your backlog.