That's not too bad a price IMHO for part-ex. This is my experience :-
I have a March 2010 Golf R - fairly basic spec and 33,4000 miles
Mint condition, full VWSH on time and no mods or remaps.
Part-ex prices I was offered over the last few weeks were as follows. I would add that these were against already discounted new cars (a new R at VW and a new RS3 at Audi) :-
£16,000 VW Dealer
£16,500 VW Dealer
£16,500 Audi Dealer
£18,500 VW Dealer
£19,000 VW Dealer
£19,000 Audi Dealer (but rang round his friendly 2nd hand traders to get this after the partner VW dealership offered £16,000

)
The lesson I learned was that VW dealers did not seem to want my car at the current time (maybe too many other part-ex's due to 1st Sept)
The last two quotes were after a fair bit of haggling with the dealer and I'm pleased to report that Lee (evo1986 on here !) from whom I got my current R gave me the best VW deal.