No cooling off period for contract signed on traders premises.
If you did not advise of how you were going to pay for the contract, as payment would be fundemental to the contract being, fully valid, I would think you could now go back and cancel. The contract you signed could be seen as being an application only, with you needing to come back with payment details, before the contract could be concluded.
I am not a legal expert, but this is my understanding. Go back to cancel to see what they say. If they have any problems, tell them that you will refer the matter to Trading Standards. You can contact TS at your local council authority or via Consumer Direct.