Probably the windows or mirrors, (assuming no loose bodywork) My old Blob STIs used to suffer from it over 100mph, but strangely the Hawkeye doesn't at all (?????)..
Wouldn't think you'll get that much for the plate, unless of course you find somebody with an 07 scooby and the initials S.V.
My girlfriend bought me MC55 WRC because my initials are M.C. and i think it was somewhere around £250, as were all the plates with SUB and WRC etc. on them. Then again I do think they have all come down in price - so yours possibly was £470 when it was purchased..
Cheers,
Mark