There is unlikely to be any penalty apart from reasonably minor administrative charge. The reason is that your rate is better than the current prevailing variable rate. If your rate was higher than the current prevailing variable, then you would be charged a breakage fee, which is a function of the rate differential and the number of repayments left on the fixed term.
The terms and conditions of breakage will be on your mortgage document, alternatively ring them and they will confirm (hopefully) what I have said