I've had to "negotiate" my way into the Glasgow lounge, to my surprise, when on a full C ticket and the lounge, I was told, was full. I got the impression there would have been no questions asked if I were a cardholder.
The negotiation comprised, on being told that the lounge was already full, me asking if I could go in, get a drink, and come out and drink it on the public seats outside !
All the BA Executive Club questions you can ever imagine, and a few million more, can be asked (or have already been asked) here:
http://www.flyertalk.com/forum/forumdisplay.php?f=446