Have you forgotten that the skids were quite damaged, so I guess getting a sample from a level airframe would not be as easy as you seem to think. Yes of course they'd know to level it, perhaps that's just what they did when it got to Farnborough.
They'd need to have checked the fuel contents in all tanks before removing the a/c from the roof otherwise fuel might have leaked in transit and they'd never have known. No way would they have used a monkey with a wrench to drain the tanks. It would have been an aircraft engineer licensed on type who would have known which components are normally level (i.e. spirit level) and then emptied all tanks in the appropriate way. Anything else would have been gross negligence. The engineer would have drained the tanks and felt and smelt around for spillage, if any, then replaced drain plugs. If they'd have had to cut to access the plugs, that's what they'd have done. Fuel is such an obvious potential reason for an a/c to force-land they would have seen to it that this was done with the greatest care, even if they'd have had to work for three days.