You have to decide whether you need or want liquidity for the future to make home improvements etc... without having to later release funds from your property through remortgaging etc... i.e. might you need cash in hand one day and how much?

Purely on a financial basis you will probably be much better off switching from the ISA to mortgage overpayments as ISA's are unlikely to pay interest rates higher than your mortgage rate and reducing the capital will take literally years off the total mortgage term, saving you £000's in interest.