There was a suggestion on the original Shoreham thread that this could have been to minimise taxi time as the Hunter hangar was close to the threshold of that runway and it would have avoided a lengthy and time consuming backtrack. Provided the performance was valid (no idea whether it was or not but I have no reason to assume it wasn't - I suppose a Hunter person could work it out), there's no issue. I have done many downwind take-offs with no issues in that respect (albeit not in Hunters).
Remember, any owner wants to minimise engine cycles for cost so ten minutes saved here and there could be significant over time.
A downwind takeoff invariable works out fine, but once in a while it all goes horribly wrong. A bit like getting airborne with runway behind you - if you feel you can justify it then go ahead. That said, it probably takes all of 3min to backtrack at N Weald.
If it was to save taxi time then was it norm to land on Rwy 20 even accepting a slight tailwind?