Well it's always going to pay it off faster, which is usually a good thing, however there are caveats.
You need an emergency fund, as you can't always get overpayments back.
If you have higher interest debt elsewhere, pay that off first.
If you are planning a big purchase that you might borrow for, you'd be better saving the money for/towards that.
Check your rate. The rule of 'saving thousands in interest by overpaying' no longer applies in a lot of cases, overpaying could actually cost you money as your mortgage rate might be so low that you can get more interest by saving the money instead - ours is under 0.5% so what would be overpayments go into a NS&I account that pays 1%.
However, only do this if your income is secure because if you needed to apply for universal credit and it built up to more than £6k, you'd be expected to use this money to live on, which you wouldn't if you'd used it to overpay your mortgage, or spent it for that matter.