I don't understand this reply!! You paid £17K for a Cooper? And a dealer has offered you £19K? Tell him he can have mine for £18K - I only paid £13K!
Seriously, if anyone has been offered a trade in price by a dealer on a Cooper, it would be very interesting to see the initial depreciation. My guess is the waiting lists are not so long that people will pay over-the-odds prices (the above example being an obvious exception).