The Firesign Theatre
-- well, 3/4 of them, as Phil Austin did not make the trip -- performed in London last October, and their half-hour show was just rebroadcast on Mark Thomas Presents
on BBC Radio 4. I think you should be able to replay it for a couple of days at least -- I don't know how long they keep the archived shows up for replay.
If you're a fan of the Firesign Theatre the way I am, you'll want to hear this show, although I'll admit that there's little new in it. What they did was put together a sort of greatest-hits show and rewrote some of it for the UK. They were IMO somewhat hampered by not having Austin, so the signature "He Walks Again By Night" Nick Danger
section had to be done by one of the others, for instance. Still, while the routines are the same general format, they've all been updated, and I had a good laugh.
It seems to me that the Firesigns would make good Special Guests of a Worldcon. They have fannish connections, after all. I reckon the only issue would be whether they had the time and the convention could afford them. I wish I'd thought of this back when we were considering our Guests of Honor for ConJose.