Originally Posted by
Kiwi Flyer
The exception to the above advice is if travel is on separate tickets. Then you'll need to clear immigration and customs and recheck in. In this case 2 hours is a bit tight (especially if travel is after UA moves to T2 when a change of terminals is also required).
Iīd have to somewhat disagree with this. You donīt need to a BP to re-clear security at LHR and the OP should be able to get their BP from UA at the transit desk.
The only exception would be if OS refused to check the bags all the way to IAD in which case the OP would have to clear immigration/customs and check in again. In that case I would indeed recommend a much longer connection.