Reading through all the posts now, it seems I either will or won't be travelling in a lounge bus and will or won't be dealt with entirely (immig/baggage/security) in one midfield terminal... ? Again - United from Heathrow and then Ted to Las Vegas.
My biggest question is whether to change my seat from the rear of the plane or not.
I guess I will just have to roll with it and see. 2hrs should be OK if the Gods smile.