Trading in for greater or lesser value, you are going to be out of pocket. Most dealers advertise a car with a contingency to drop off a few quid for a straight sale, without trade-in.
You won't get this benefit when trading in, but will presume you have got a good deal, from the price they are offering you against your car, but which also has the contingency diluted in to it.
Selling premium cars (+15k) privately, limits your market, as many buyers would prefer a dealer "guarantee" to run with these prices.
Me personally? Get the car fully valeted and advertise on autotrader or elsewhere, with a photo.