Queen never did perform 'Under Pressure' on 'Top Of The Pops'. That is why it does not exist!
The track 'Under Pressure' did air twice though; The first airing was to some dancers called 'Zoo' and the second was to the official promo video.
Zoo (Dancers) - Under Pressure, 19th November 1981, Steve Wright
Queen & David Bowie Promo Video - Under Pressure 31st December, 1981, Mike Read
You can read all about this at the official BBC Top Of The Pops site. http://www.bbc.co.uk/cgi-bin/totpperf/search.pl?type=totp1
So sorry guys - this is another one of those Queen myths exploded.
"Listen to them. Children of the night. What music they make."