you wouldn't be able to access the BMI lounge at T3 from T1 as that lounge is located in a domestic arrivals and departures zone, so any passengers who had cleared security at T1 could not then move over to the zone where the BD lounge is. Also, direct airside transfers (those passengers who are in transit and have not 'entered' the UK, would not be able to use that lounge as it is essentially in the UK and outside the immigration restricted zone.