I'm presuming the concern the bank has is that in the current market, the value of the completed house is not sufficent to secure the loan. However unless you talk to the bank, you'll never know
Suggest you ask the bank to explain, in writing, the reason why the facility is being withdrawn and request a meeting with the relevant branch manager and not just the mortgage advisor in the bank. If you are not satisfied with the response you get there should be a formal complaints process for you then to follow and after that, if you still have not got resolution to your satisfaction, there is always the ombudsman, but he normally will only deal with a complaint once all other options with the bank are exhausted
On the basis that you have the finance to complete the build anyway, that might be a cheaper approach initially then getting a solicitor involved