There's Windsor Castle near the green area of the center (Windsor Grey Park). And some related military installations close by. Easy to see why it would be jammed...
No. There wouldnt be jamming of civilian navigation right next to one of the busiest airports in the world. Most likely cause is lots of gps warming up at lhr.
No. As another commenter mentioned, if that were the case you would most likely see it at other airports, and you don't. Additionally, the data that GPSJAM shows is filtered so that an aircraft isn't considered to have "bad" accuracy unless it has first reported "good" accuracy recently.
London is the busiest "airport system" in the world and handles a lot of international traffic, so there might be something unique about it that you don't see somewhere like Atlanta.
That being said, it seems like London only has interference recorded today and every other day is green. I would imagine it's more likely to be a quirk of how the data is recorded rather than geniune jamming.
Not sure how you measure busy, but Atlanta handles way more passengers than London. It may be that London has far more flights due to the use of smaller aircraft however.
Do they use ground mlat there? That is the only reason i can think of that you would get cold gps transmitting. Lht was first operational ground mlat site, but i cant find a current list.
This gpsjam site doesnt have much info on methodolgy. Seeing the data would probably show what is causing this issue.