Several things need to be considered as to why this idea doesn't seem to appeal to the UK government.
For a start, the UK does not have any UK-owned production car companies.
At what age/value would a car be scrapped?
How many ordinary folk could take up this option for a £2,000 cashback on a new car from the government?
I have recently changed my 4-year old, low mileage car for another similar model. The trade-in value was £5,000 and, even with a very good deal from the dealership (and with a manufacturer's reduction), I still had to find around £10,000.
In these uncertain times, everyone is faced with the spectre of losing their job, so it would hardly make sense to scrap their current car if it is reliable, probably have to take out HP payments and risk repossession, and end up with no car at all.
When the economy appeared to be on a roll, people in the UK were frequently pilloried over their excessive credit debt. So it would hardly make sense to extend the debt during a recession.