Originally Posted by The _Banking_Scot

Hi,

on arrival at LHR T5, from MAN, follow the purple signs for flight connections to T2,3,4 ( there are separate signs for transfers within T5)and get a bus to T3. You will clear security at T3 flight connections ( and may go to the AA desks for Document check) . You could also go landside at T5, take the tube/train to T3 and go through security at T3 but this is likely to take longer than using the flight connections
If your flight to the US was from T5 then for DOM-LHR t5-INT there would be no further security at T5

Regards

TBS