"We want to honour the contract and complete the sale but if we cannot secure the funds is it considered a breech of contract? "
You (a) buying the property and you (b) getting a mortgage to buy the property are two entirely separate ( in legal terms) matters. Whatever prevents you from buying the property, prevents you. Be that for whatever reason.
So - the real question is can you fund the difference between what you were hoping to borrow and what you can now borrow? In my experience, people generally can but prefer not to.
Is it a legal excuse to say to the builder, I cannot complete because I do not have the money? No - but its certainly worth a go as a bargaining tool to seek a reduction in the price.
mf