Define a Fallback Locale. Media Entries need 1 image per locale (see image). If you don't set the Fallback Locale for 'uk' it simply returns empty. Other option is to go into your Media Entries and upload the same or different image for 'uk'.
Settings -> Locale -> Fallback Locale
After you set up the Fallback Locale there will be a little reference icon next to your uk media entry and the payload will have fields again :)
without fallback and separate locale image: