Any idea if any UK banks offer a mortgage to buy a property in Poland?
a) it's more risk to them as they're operating outside the market they know well, so they're going to want a higher return, i.e. the mortgage is going to cost you more;
b) in the event that things go pear-shaped and they need to repossess, they don't know what they're doing or how to do it, i.e. there's more risk to them and thus the mortgage is going to cost you more;
c) there will be currency risk involved (as you'd be borrowing in GBP but the asset securing the loan will be valued in PLN), so the bank will want a higher rate of return and the LTV ratio is going to be a lot lower;
d) it's going to be a humongous pain in the cock from the paperwork viewpoint.
You're really a lot better off going to a Polish bank. Either that or releasing equity you have in your UK property by remortgaging there.