Senior Moderator, Moderator: Community Buzz and Ambassador: Miles & More (Lufthansa, Austrian, Swiss, and other partners)
Join Date: Jun 2005
Location: 150km from MAN
Programs: LH SEN** HH Diamond
Posts: 29,516
I'm not familiar with T3 but I've seen at least two Flight Transfer signs in T1. The Flight Transfer for passengers arriving in T1 is on the left just before passport control if you are coming from 2x-3x gates or opposite the escalator coming down from the lower numbered gates. Transfer passengers departing from T1 seem to come up from the lower level in the same lift that takes you up to the lounges.
Transfer without entering the UK must be possible but like other posters I have no direct experience.