Findmypast - Passenger Lists Leaving UK 1890-1960

The National Archives (UK) Outward Passenger Lists for voyages departing the British Isles 1890-1960  (BT27) for Australia, Canada, India, New Zealand, South Africa and USA detail 24 million+ records. The database covers many arrivals into Australia that are still unindexed here. Passengers listed include migrants, businessmen, diplomats and tourists. It is also available at Ancestry.com as UK Outward Passenger Lists, 1890-1960.  These two commercial sites offer a range of subscription options. 
TIP: Findmypast and Ancestry.com are available for free, onsite, at state, public and family history libraries. Check what might be available onsite at a library near you.