What you need to remember is the dealers only have so much margin to play with. If they discount the car, their margin will reduce by the amount they knock off the price.
If you take out a finance deal, they will earn commision based on the amount of money the lenders will make on the deal, therefore increasing their margin. With this increased margin, you can possibly negotiate a better discount than you would if you were paying cash. However, when you add up the total amount that you pay back over the term of the deal, even with a better discount, it would still be more than paying cash on day one
I'm sure that they could lend you money at a rate of 0%, but this would cost them money and therefore reduce their margin. Someone, somewhere has to pay the interest on the borrowed money. If they did this, it would be unlikey that you would get any discount off the screen price.
I also doubt the any dealer would lend you just £14k with no deposit to drive away in a £28k car. It's value would drop by 20% the second they put the tax disc in it, leaving you with a massive amount of negative equity straight away. Even if it was possible to do that deal, personally I wouldn't want to spend £14k over three years and have nothing to show for it at the end.