Plus, if I can just add to the previous post by the never-ending RJ.
British Airways did not build their own terminal.
Terminal 5 was built by the BAA for the exclusive use of British Airways. Athough that wasn't assured when it was given the green light for construction, which started in September 2002.
Terminal 5 has 50 gates. British Airways has grown it's network significantly since Terminal 5 was originally planned hence why they also use Terminal 3.