It also depends on how much traffic there is. About 1/3 of the time.. I have experienced an Air Traffic Control delay going into LHR. Plus taxi time.
Like others have said, rush. You did not indicate which class you are in but the closer to the front of the plane the better. After an overnight flight, folks can be slow deplanning. And have your stuff organized when you touch down.
There are certain times when the crowds at LHR are intense.
Also, there is no consistency on UA printing board passes for the LH. And there are times when I have had a boarding pass printed by UA and once landed LH required a different one. One thing you may want to do is to check in for the LHR-TXL flight on the LH website.