Hi,
Unfortunately this is one of those situations where, as frustrating and annoying as it is, there is nothing you can do. You can complain but you're unlikely to get anywhere, as you had not signed on the dotted line. Had the agreement been signed by both parties already then the bank would have been subject to a binding agreement under UK law.
However, the bank retains the right at any time to withdraw an agreement before the contract is finalised. This is why you'll usually see stuff like Agreement (Subject to Status) on these forms. It's an indication that the bank or lender has initially agreed to lend you the money based on market conditions and your financial situation at the time of application.
Should any of these conditions change before both parties have signed a credit/loan/mortgage agreement then both the lender as well as the customer have the right to cancel any application. It's designed to protect both the bank and the customer.
Rough I know but rather than wasting your energy trying to get a company to stick to their original agreement when nothing's been signed I'd shop around and see if you can find a similar deal.