What I'd do is first negotiate the price of whatever new car you want, getting any and all discounts via various methods, and ONLY then after that you bring in the scrappage available, especially if your current car is valued well below the scrappage you can get.

Don't talk about it before the main negotiation is done as they'll just factor that into the discount by offering less of one. If they try and say the offer already on the table cannot happen (i.e. they can't offer as much of a discount) if you take scrappage, then it's probably not worth it. To me, the often large differences between certain models in the scrappage they will offer means that one is selling far better than the other, even when taking into account the difference in the list price.

Note that some (e.g. Mazdas) only offer such deals on lower CO2 emissions cars, i.e. not the larger-power output and/or 4WD/auto cars.

I’m interested to know whether they offer a real discount when most manufacturers offer discount off a new vehicle anyway.

When I was sourcing the uncles new car we looked at the Kia Picanto which had a £2000 scrapage discount. That made the car in base colour £7895.

Alternatively I tried getting a discount and taking advantage of the PCP contribution (pay off immediately) of £650 and still being able to sell the Fiesta to WBAC. Dealer said no discount so the price payable would have been £8835 (£410 for Fiesta).

Tried Carfile. PCP contribution the same, sell to WBAC the same but Carfile were offering £816 discount. £8019 to pay.

So basically the cheapest way to get a new Picanto with no hassle was the scrapage way.

But other deals on different cars may not work out the same.