But what use is 0% to them ? Why wouldn't they just take the full amount there and then rather than have to wait 2 years.
The dealer doesn't have to wait two years, the minute you sign the documents and they are sent off to the finance company, they are paid out in full and you enter an agreement with the finance company - not the supplying dealer.
They may have to sell £150k of finance per month (for example) so your £3k loan helps them on their way to ensuring they sell the minimum amount of finance required.