£6167 of what you paid is VAT that goes to the government. You need to work out depreciation on the price without the VAT. So it's £14k.
When they sell the car, the buyer will need to pay VAT on top of their price (but the price will be displayed to them as the total including VAT).
Also - for comparison - if you'd got the car on a monthly finance deal then what would the payments have totaled over those 18 months