It doesn't matter what is in your contract to a degree. Your employer has to, by law, afford you the opportunity to take your leave, namely 28 days per year, including bank holidays.
They should not pay you for them at all, as that is not giving your statutory leave entitlement and they would be breaking the working time directive. You should be made to take them. That said, some employers do do it and if they are going to pay you it's at your pay rate, not SSP.