This type of mortgage is regulated by the Financial Conduct Authority and there are varying types from many financial institutions, charging vastly differing fees and interest rates, with differing rules about making overpayments, taking payment holidays, redeeming the mortgage etc.
Since the credit crunch lenders have changed their policies on whom they will lend to in different circumstances and how much they will lend based on your income and size of deposit. They change their policies and the rates on offer very regularly and so getting advice from a whole of market adviser can save a lot of time and trouble as well as money.