This new fangled thing called Google gives you the following if you type in
"british airways bag drop terminal 5"

Bag Drop desk opening times

Premier, Gold and Silver Executive Club members: From 05:00 on the day of departure

First, Club World and Club Europe passengers: From 05:00 on the day of departure

Euro Traveller and Domestic passengers: 3 hours before your flight departure time *

World Traveller and World Traveller Plus passengers: 3 hours before your flight departure time *

*The earliest Bag Drop opening time is 05:00.


They even have a page for terminal 3.