There may be small amount for legal fees, particularly if you transfer between lenders. But you won't be doing the same due diligence as the initial purchase.
MSE has an overpayment calculator, so you can work out how much you'll save;
https://www.moneysavingexpert.com/mortgages/mortgage-overpayment-calculator
From an interest perspective, the first £1,000 each of interest is tax-free, so if savings rate>mortgage rate, savings win (and have the advantage of greater flexibility).
I still think you need to work out your life plan. Although capital is at risk, historically, the stock market has returned more then 1.84% when dividends and capital growth are considered. There are tax advantages within a pension (as you can take 25% tax free on retirement, and might get NI savings though salary sacrifice with your employer).
I'd at least consider these a hybrid of the mortgage overpayment/increased pension. Mortgage overpayments alone is low-risk, but unlikely to be the best use of surplus cash.