Apologies if this is old news, but I noticed yesterday that the Kings
Cross departures board now shows ECML Thameslink departures from St
Pancras:
https://www.flickr.com/photos/recliner/49423851683/in/dateposted-friend/lightbox/

I can't recall other instances of UK rail stations also showing
departures from other, adjacent stations, but perhaps there are some
that I'm not aware of.