I haven't done it (or tried to) on BA, but I've had the good fortune to ride in cockpit jump seats on Qantas 737s and 767s, a Jetstar A320, a Druk Air A319 (sitting in the cockpit while weaving through the mountains on a landing approach into Paro--called the most difficult landing in the world by Boeing--was one of the coolest things I've ever done) and a GMG Airways Fokker. I had a JS boarding pass for Vietnam Airways once, but ultimately got a regular seat. It's definitely easiest to get in the cockpit--whether to see it or ride in it--on smaller Asian carriers. I've walked into a Qantas 747 cockpit before, but got a bit of a lecture for doing so (and was never allowed to jumpseat in a 743 or 744--I knew better than to ask on the A380). Note that I do work for airlines and have contractor IDs with airside access for several carriers, which makes talking my way into the cockpit easier (though on occasion I've ridden jump seat without showing any ID).