Essentially, what the mortgage company does and what you need to do are two separate things, especially for an old building.
The mortgage company ONLY needs to know whether, if you default, they can get their money back - so they need to know that the property is almost certainly worth the amount they are lending. if you are putting down a substantial deposit, it really isn't that helpful to know that e.g. the house is worth £200k if you are paying £250k with £50k deposit. They won't care whether it is cold, damp, will need substantial work, costs gazillions to heat - they just care whether it could be re-sold for the amount of the outstanding mortgage.
That's why you need a separate survey for you - is it a money pit? Is it damp? Do the electrics need work? Will the roof need re-doing in 5 years? What work is needed instantly, and what might wait a few years? Is the idea you had about using the loft feasible, or is there an obvious reason why it can't be donw?
Because of the two purposes, you need both surveys. DEFINITELY don't just rely on the mortgage valuation ... remember that in 5 years it doesn't matter to the mortgage company that the house is worth 10k less, because they will have had that amount of money from you. However for you, the fact it now needs a new roof is significant...