And may I be the second to welcome you to FlyerTalk, and indeed to the BA forum. Welcome CD747, may we see plenty more of you here, it's good to see you here.

And I can only endorse the other replies, you should not have a problem so long as you have the CE boarding pass to hand when you arrive. It's quite a nice lounge (there are a few treats on a special table reserved for BA passengers) but it is not run by BA. In Terminal 5 you should have no problem, and I suggest you head to the South Lounges for Galleries Club lounge on the top floor. The exception would be if your flight is going from Gate B - which does happen from time to time - in which case the lounge there is probably your best bet.