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. link
So sorry guys - this is another one of those Queen myths exploded.
No worries, I have this in DVD quality as well as Another One Bites The Dust (Legs & Co.) should anyone want them.
They were both re-broadcast on BBC4 FOUR HD recently in nice quality.