Hello, I hope the following is of assistance.
Usually the tax disc remains with the vehicle and the dealer will get any refund due, the dealer will have used the refund in the equation for the value of the car.
If you want the tax disc for a refund then you need to state this to the dealer before agreeing on any deal this prevents any misunderstanding. The main reason for this is some cars can cost nearly £500 to tax so if this were deducted from the price of the car it would be a significant difference.
I hope this helps but if I can assist further let me know and I'll reply as soon as I'm able to.