Groundloop and Archimedes are indeed correct - the aim of the exercise was to try to diminish the prospect of ecological damage by setting the fuel on fire, coupled of course with the opportunity to conduct unusually realistic bombing practice.
Curiously enough, and without wishing to incur the wrath of the Mods for threadcreep, I recall that one of the Observers in the Buccaneer NAS involved with bombing the TORREY CANYON, and gained much newspaper publicity at the time, was one Bodger Reardon, who had a rather unusual claim to fame. In 1963, he was the first, and I suspect only, "Looker" to convert from Sea Vixens to Buccaneers by means of an onboard conversion course, whilst serving in HMS VICTORIOUS's Air Group.
Almost by chance, it had been discovered that Bodger had such a long torso that he could not sit fully upright in his "dustbin" seat in the Sea Vixen, and therefore would virtually certainly have broken his back if he had had to bang out. VICTORIOUS was at the time half way to Singapore so the solution was an immediate transfer from 893 NAS to 801 NAS and a very intensive and successful conversion from Vixens to Buccs - a considerable achievement for all concerned (since 801 squadron had embarked just days previously to take the Bucc to sea for the first time in an operational squadron), not least for Bodger who was very young and junior at the time - weren't we all!
Jack
For much more detailed info from participants and - interservice banter! - See Torrey Canyon - 18 March, 38 years ago