£16-17k in the used market is retail money - a dealership is going to want to make at least £2k out of the transaction, so p/x of £15k at 3 years old and average mileage is more likely (vanilla 5 door GTD GFV is around £14400), maybe £15600 (£600 in lieu of your options - options are nigh-on dead money at p/x time) if you're lucky.
You'll struggle to sell that car privately at £16k - the vast majority of people buying a car at that pricepoint want to do it through a dealer for reassurance on such an expensive buy, warranty coverage (dealer will most likely give a 1 year used warranty with it) and access to finance for the car - as well as being able to part ex what they already have.
A good part-ex price and £1200 discount is the best anyone seems to be getting directly via a dealer right now. If you want £3500 off that car then you'll have to go through a broker, and as you have no p/x then I wouldn't mess about haggling with dealerships.
I'm sure you'd have had your company GTD a lot quicker than you'll get one funding privately - you're probably talking Jan/Feb delivery for an order placed now, which should easily be delayed to the "14" plate if that is what you're after.
You might struggle with credit card financing though. Seeing as most credit cards take a chunk of the transaction between 3 and 5%, the dealerships impose an upper limit on how much credit card can be used to make a purchase, or be used towards a purchase. For my supplying dealership it is £2000.