Is that a price for the dealer to just buy the car from you for cash?
If that's the case, don't expect to be offered anymore than he would pay at auction for one. Why would he pay anymore for a car than he absolutely needs to? If you've gone to him, you are automatically on the back foot when it comes to negotiation.
You would be better off advertising it on Auto Trader at about £25,000, with a view to accepting a bit less. It will probably still be bought by a trader, but one with lower overheads that will not be so greedy when it comes to making a profit on it.
If you paid £31k for the car, it will loose the VAT the minute your name goes on the log book. That makes it £25,800. Take off another £3000 for depreciation and your at £22,800. So to be fair, he has offered you a realistic amount.