It's an ATC legend. The TSR2 was only flown from Boscombe Down intially, then it went up the Warton.
The Vicker's contingent at BAC wanted the a/c testing done from Wisley, while EE wanted it done from Warton. In the end the Government said it had to be done from Boscombe.
If you read Roland Beamont's books he say's Wisley's runway was too short in the case of a brake failure as the a/c would be the other side of the A3 before it came to a halt. Presumeably that was if the a/c survived the steep drop from the runway to A3 level.
Rgds
ETSD (ex-11F Sqdn ATC)