Once the new T2 is open, T1 is coming down to allow T2 to be extended. Indeed the North wall of the new terminal bulding looks to be temporary.
When you say T1 do you mean all of T1 or just part of it? I'm not familiar enough with LHR as I used to be and I'm curious to know if they are going to knock the entire metal tubing that is T1 and stiffling hot during the summer. The memories of flying back and forth to Ireland in the 90's and early 00's.