It's actually been possible to enable 2FA on Amazon.co.uk accounts - albeit indirectly - for some time now (though being able to do so directly from the .co.uk site is welcome): https://nakedsecurity.sophos.com/2016/06/30/setting-up-two-step-verification-on-your-amazon-account/#comment-4565335

As an aside, this also backs up your point about Amazon now being "a maze of interconnected legacy systems".