For a very long time, T1 did not separate passengers arriving from/departing to Ireland. Sliding glass screens were added but only maybe in the last 2/3 years or so of the terminal's life. Many times I almost missed a flight from being held up while the sliding glass doors were closed to let arrivals through!

I don't know T3 that well (old T1, current T2 and T5 are the LHR terminals I mainly use), but from recollection aren't check-in desks and arrivals on the same floor? How does this fit in with keeping arriving and departing passengers seperate? Or is it not deemed an issue since it's all landside? And is it only possible to walk from T3 check-in to T3 arrivals by walking outside?