I am not sure whether loans of up to £50k are still widely available for this purpose. It depends on your financial status.
Some relatives of mine, invested in a number of such properties, after the last recession. They obtained a business loan, working with one of the main high street banks, that they held one of their accounts with.
You may therefore wish to contact your bank first to see if they are willing to lend.
As a by the way. Just because you bought a property 25% below its previous market value does not mean that banks will look upon this, as you holding 25% equity. As far as they are concerned the property is only worth the price you paid for it.