Before any mortgage lender will agree to lend to you they need to know that the property you are going to buy is worth lending on in the first place. Why is this so? Well, in some respects it is to protect you just as much as it is them. What you have to remember is that the Mortgage lender wants to lend money, it’s how they make theirs, but they will not lend on a property that cannot be sold at a later date.
It might be a “worst case scenario” but should there be any long term issues with your payments and you have to sell the place then if it has no value the lender will be left with nothing. As the average house price in England is £248,000 even if you had half left to pay that means the lender would be down by £124k, which is a lot.
The other question that you need to ask yourself is do you really want to buy a place that is not fit to live in or it will need a lot of work done to it. Whilst there is a good market, and it can be cheaper to buy a “fixer upper” it is not always possible to get the lender to see it that way. This is why you need a Structural survey done of the place to see if it is sound.