"Onboard fire" could as easily be an official fib as an accurate description of what actually happenend. The MoD and Govt would be most reluctant to admit to a shootdown until it becomes impossible to prevent the knowledge getting out, not least to prevent the inevitable triumphalism of the baddies, and the result of a missile (be it rocket, lucky shell or whatever) strike would very likely result in an "onboard fire" so its not so far from the truth either way.
Anyway, I suppose it was a MR2, and not, well, another sort of Nimrod?
They surely wouldn't want to admit losing one of those out in wild n' woolly land, with the bits strewn all over the countryside...