I have always gotten access to the EI lounge in LHR - was actually denied access at the BA lounge once and told that I could only use the EI lounge. However, I am PLT, so maybe that's why.

You didn't miss much. It is a windowless room with uncomfortable chairs, no restrooms (you have to go out into the terminal and do your business with the common folk) or decent food. The only saving grace was the bar. I'll be back soon and not looking forward to it. I am hoping to beg my way into the BA lounge as we have nearly 4 hours there! Or head back to T3 to the AA lounge.