I attended Grad Bash last night and got to exit on the road directly behind the Gringott's show building. I took a picture of the massive AC and Heat units from the back point of view, an exhaust pipe that was pumping out steam at the 2:00am-2:30am time frame, and even a portion of the outside line queue that looked almost 100% finished from my point of view. There was still some construction equipment in the space behind Kings Cross and in between the outside line queue and the road I was on, but nothing that couldn't be easily cleaned up for the speculated June opening date. I'm having trouble posting the pictures on here from my iPhone, but I hope this sheds some light onto the mysterious date that is the Diagon/London opening date.