When are you flying?
If before June, OS arrives into T1 and UA departs from the same terminal.
As others have noted, you will re-clear security and two hours will be plenty of time for the connection.
From the LHR website: how to
connect T1>T1
From the UA forum:
http://www.flyertalk.com/forum/unite...2-onwards.html
If you are flying in June or after then, then UA will be the first carrier to operate out of the new T2 and other *A carriers will follow.
So, if you are flying early in June, you will have to go T1>T2.