That's an interesting position to take; I appreciate that many international organisations have taken it as a sop to US support.
However, does it actually serve our users well to give them 'en-GB-oxendict' when we say it's 'en'? Is Oxford Press's attempt to fuse British English with some (but not all) American English spellings actually closer to what normal people use in India, or Australia, or Nigeria? The purpose of this is to greet our readers in an interface that is most familiar to them, and especially with a top-level generic language code to pick something that is as neutral as possible. (Note that we don't historically do this very well; 'fr' is almost exclusively French French, and so on; we should probably fix those, too.)

@Jdforrester-WMF Re: the actual code changes: Having en as separate from both en-GB and en-US means that en is international (Oxford English), which means it uses the -ize suffixes, but essentially all other British spellings. Instead of changing those, we'd want to change places where it would say color, center, etc. to colour, centre, etc. Also, we'd want to change all date formats to ISO 8601 for numeric ones and RFC 2822 ("DD Month YYYY") for spelled-out ones.

Do the language codes in MediaWiki's list match ICU locale codes? They certainly appear to, but then we are overriding things like date formatting, so perhaps they shouldn't thought of as strictly the same thing.