Imperial Airways even. I was given a book of BOAC / Imperial / BA (the first one) advertising posters. There were a few saying "Fly the world by Speedbird" or "it's a smaller world by Speedbird" so I guess it was retained as it was thought of as much more than a logo. Curiously there was a 1929 poster from Imperial Airways with the strapline "All Imperial Airways planes have four engines for security". I believe that has been swiped by another carrier...

You can look here for a list of all call signs. Aer Lingus are shamrock, Pan Am was clipper etc...