SharePoint Regional and Language settings for all sites.

There are several things here, but first the bad news: there is no way to change the site language setting after the site has been created, and en-gb is not a supported language.  You can choose that setting, but you'll still get en-US.

On to better news.  The site's base language and the site region setting are only defaults, they are used when the user doesn't have a setting.  The user's language setting is in their M365 profile.  If that is not set it will use the browser language.  Provided that the site supports the users language setting, and most sites have at least basic support for all 50 languages unless you remove them, the user will see the site's UI in that language.

Region settings are a bit different.  Region only affects date and numeric format.  En(GB and en-US differ only in date format.  Again, the user profile can override the site default setting, but only if you check the override setting.

What can you do as an admin?  If you don't want anyone to see the site UI in any language other than English (en-US) then you can delete all the other languages in the site's language settings in the advanced settings.  If you want users to have a choice, then you can provide their initial settings in their profile using AAD, which syncs with their profile.