You won't know what deal the dealer can offer until you ask. Having a nice, retailable car to px should get you a good trade in price, as the dealer knows he should be able to sell it quickly and get his money back. However, as we all know, it's the price to change that matters.
I have sold all of my cars privately recently, but then I had kept them for a long time, so they were down in the £3-4000 price range. Buying outright will get you the best discount off the new car from the dealer, but cash is not always best. They like to sell you finance to boost their commission. You could always play along, getting a better deal due to having finance, but then change your mind and hand over your debit card on the day of collection.
We have had the Mk6 for just over 3 years now and I have considered that it could be time for a change, however I'd like to think that it is in exceptional condition and only done 33,000 miles. To me it's too good a car to let someone else have just yet.