Ok, so let me try some math.
My car a middle range turbo diesel hatchback is worth £650 apparently. Say I can sell it for that and buy a used turbo diesel Mondeo hatchback for £3k, I am out of pocket £2,350 and I have replaced my old middle of the road hatch with a more modern one.
If I still want a middle range TD hatch but am looking at a new one surely I will be paying some £10k for a new turbo diesel Mondeo hatchback, but I am then getting £2k off my old car leaving me £8,000 out of pocket.
It only makes sense if I buy a very budget new vehicle and get a good discount on the price but I don't want such a vehicle, I want something like I have at the moment.
You say, "this £2K *could* swing it into buying a new car." and I agree. I just can't see it swinging people like me