Hi RMCF, this won't affect the mortgage process. The things you have to have in place to drawdown mortgage monies are life assurance, direct debit mandate, house insurance, solicitors undertaking etc.
Whether the BER rating gives you scope to negotiate with vendor would depend on house in my opinion. If you're looking at a 1920s house then the BER rating is going to be poor and you should expect this and I would say little room or negotiation. It would be different I think if you were looking at a new build.