A post office combined with a shop is a commercial premises and will probably have a restriction on use as a commercial property only.
The first question to ask is: will the local planning authority be prepared to change the use of the property from commercial to residential?
Provided that change of user can be achieved, it is almost certain that the lender will require the property to be converted into a normal residential property before completion of the mortgage.
On this basis, it may be necessary to arrange short term funding in order to purchase the property and then convert it, before the mortgage is completed.
The lender’s valuer will probably want to see a copy of the conversion plans to assist in its valuation as a residential property.