property "not lendable"

Urgent Advice needed! - Banks saying property is "not-lendable" and we have already paid large deposit and exchanged contracts - what can we do if we can't secure mortgage without any fault of our own?

Quick Summary -
We exchanged contracts on a new build, now banks say the property is not lendable, we have already handed over large deposit. what option do we have, other then being forced to lose deposit or buying 100% cash if banks offer no mortgage.

Details -
We exchanged contracts with the Builder in August 2016 with a small Payment. Completion date of property was expected Oct2017. We made further payments since, adding up to 20%. In August 2017, we were asked to apply for mortgages as completion was nearing. When we applied to banks, the first response is now a shocking "applicaiton denied as surveyors recommendation is the property is 'not-lendable'. "
the contract with the builder says we lose our deposit. We only spoke to the banks last month, so we have not had a mortgage in place, as we are first time buyers. The Mortgage brokers are trying to find other banks. But it still stands that the property is difficult to lend against and that it might be difficult to sell in the future. And we were not aware of this at the start. Also, its surprising the first bank brokers chose to apply to takes the worst possible view. surely they would pick/talk to bank they know.

The reason given for "not lendable" is that the the property is in a commercial area with a Pub and casino on the same street.

What we were adviced-
We were not aware of what is the best approach, and the Builder setup a launch day where he invited two solicitor companies, and then we were encouraged to sign and exchange moments after providing holding deposit. The Solicitor we were pointed to was already there, he pointed out the various documents and key highlights of the contract and then encouraged us to sign. We assumed this is normal process for a new build launch event.
We were not highlighted the risks involved especially the ones outside of our ability to borrow. As in we knew we were very eligible for mortgage, and thus assumed that getting the mortgage would not be difficult, we also had mortgage in principle ready. But what we were completely not expecting was the mortgage to be turned down because of reasons different from our eligibility, in this case, the eligibility of the property itself.