I agree - the only alternative independent
evidence which might be acceptable to an appeal panel is probably going to be an educational psychologist's report.
Although they do not test the curriculum directly, the Verbal Comprehension and Perceptual Reasoning scores are indicative of ability/potential in English and maths.
How much weight is given to any particular bit of evidence is, of course, up to the individual appeal panel.
For a list of academic evidence, see:http://www.elevenplusexams.co.uk/appeal ... cation#b11