G-AHBO
The original question asked about the 1951 incident that involved Taylorcraft Plus D (Auster I) G-AHBO which, although owned by The Wycombe Flying Club at the time, was hired for the flight at Denham.
In 1954 an American, Gene Thompson, performed a similar feat using Plus D G-AHAI also hired from Denham.
The more celebrated/infamous incident involved Auster J/1 Autocrat G-AGYD flown by Draper who had advised the press of his intentions beforehand.