If this is your only aim, then the easiest and least complicated way to protect your money is to put it in to a savings account.
When you re-mortgage you can pay off any amount lump sum you want. The only saving you will have made between overpaying and not is of the interest you would have paid on the portion of capital you would have overpaid down. This is offset by the amount of interest you will earn on the money that is built up in savings. So for the minor cost impact you get all the protect you want without fuss, and you can deal with the hassle of paperwork etc all at the same time when you re-mortgage rather then piecemeal as you try and overpay.
In your case, not overpaying, but saving is the most sensible solution.
Thanks. Just checked and our interest is 2.34 percent.
Anyone able to recommend a savings account so I can compare? Hadn't thought to go down that route