You are right to think about what problems the short lease may cause in the future.
As you may not have to rely on a mortgage it won't prevent you buying, although it is likely that your solicitor will point out the downsides of a short lease.
If you did want to use a mortgage now to buy the property, then you would certainly find your options severely limited.
Mortgage lenders will require a minimum unexpired term in order to lend on the property, and 43 years remaining would fail to meet standard criteria.
Lenders, buy to let or otherwise, will typically require a minimum unexpired term.
For example, BM Solutions require at least 70 years on the lease at the point of application and The Mortgage Works requires at least 55 years at the time of application and at least 30 years remaining at the end of the mortgage term.
That may not only present you with a problem now if you wanted a mortgage but would mean that any prospective buyer in the future would not be able to secure a mortgage. That would limit the number of buyers that you could appeal to and inevitably have a knock-on impact on the selling price.
It therefore makes sense to extend the term of the lease before you buy the property.