In the French Pyrenees not far from Gavarnie earlier this year, I came across this unrecognisable butterfly. It was in pristine condition and so its unusual appearance was not as a result of wear. Its flight pattern was that of a fritillary but which is unclear. Heath and Pearl-bordered Fritillaries were common in the area, though several others also occurred. I wondered if this is a known aberrant form of a particular species or whether it is unique. If the latter, no doubt it would have been worth a mint as a specimen in the days when butterfly collecting was the rage!
This is close to Heath Fritillary, Melitaea athalia ab. cymathoe,
only yours is more extreme
http://www.ukbutterflies.co.uk/species.php?species=athalia(scroll down to 'Unclassified Aberrations')
I know it is not from the UK, but Peter Eeles might still be interested in it;
email address on this page - http://www.ukbutterflies.co.uk/contact.php.
Oh - I forgot - this is another excellent resource on butterfly aberrations - the Cockayne Collection at hte NHM
That shows ab. nigrathalia as another dark one. But yours is still different.