Can I cancel a car agreement after I have signed the vehicle order?

[haveaclue]

Just went to a showroom yesterday and seen a second hand car. I said i wanted it and we went through all the paperwork including getting the finance agreement approved and i have signed what they call a used vehicle order. They have taken a £200 deposit. I have since decided that it's not a good idea. They are taking my car as part exchange but can I now phone them tomorrow and say keep my deposit, i don't want the car. I know it's stupid but thats me