My experience....
I have been offered £22,500 by Jaguar for my March 2014 S3 Sportback with 22k miles, against an F Type AWD Coupe. Just waiting for an update after they do a call around to see if they can improve on price. Realistically I do not expect much improvement.
I have been using various sites to get free valuations (CAP, Audi, WBAC) so that I know what to expect. They vary between £21k and £23k.
When I bought the S3 I asked for a finance quote and it gave £17,978 as a GMFV after 3 years. Based on the numbers thus far I would say the trade in value would not be far off this. If I had taken the finance the interest would have wiped out the discount and I would not have seen my deposit when I came to change in 3 years.