Some mortgage lenders may allow you to pay off your mortgage early, however, if you choose to do so, you may be subject to early repayment charges (ERCs).
Some mortgage lenders may cap how much you are able to overpay by. Most of the time your overpayments will be capped at around 10% of your remaining balance.
If you are on a standard variable rate you should be able to overpay as much as you want.