You almost certainly will not be able to go through security in T2. I had an EI flight out of T1 a couple of days after T2 opened and tried to go through security there. I was told in no uncertain terms that I wasn't allowed to do it. I was first lied to and told "there's no way to walk airside between T2 and T1" and the lady only changed her tune to "no, it's just not allowed" when I told her that a friend of mine had done exactly what I was trying to do just the day before. She was not budging and I didn't have that much time so I just gave up at that point although I did complain to a Heathrow employee in a purple coat about being lied to. She was very friendly but basically told me that security are a law unto themselves.

However, as mentioned, a good friend of mine was able to go through security in T2 for a flight from T1 so it really depends on who is working the desk when you ask. Good luck!