How much is your ERC? Is it really worth throwing that money away and more just to see a lower balance? <does not compute>
OP, for the last 15-20 years, I've dabbled a little in what became known as 'stoozing', so named after the username of an old financial forum I used to inhabit where 'stooz' became known as one of the early proponents of doing this.
It involves doing just what people are suggesting and deliberately keeping a debt if you can profitably offset it with savings. It's mainly to do with borrowing on credit cards at 0% and zero fee and putting the money in a savings account, making the minimum payment and repaying at the end, and collecting the interest earned on the savings, or more likely, transferring the money again and just carry on. People did this with multiple credit cards and made quite a lot of money. I've not gone into it hardcore but have still consistently made a few hundred pounds a year just for the odd bit of financial admin.
Then when mortgage interest rates were pretty much nothing for the last 10/15 years people have done it on their mortgages, carried on paying the minimum on their mortgages (until very recently our mortgage rate has been as low as 0.48% and has only just in the last few months gone above 1% since 10+ years ago) and then saved or invested their spare money instead of overpaid.
At one point I made back just about all the interest our mortgage cost with £2500 in a Nationwide current account that paid 5%. We also had other savings that paid up to 3%, while our mortgage interest rate was under 1%.
Now because we're still on a lifetime tracker, it's all changed and come next week we'll probably be at the stage where I can no longer beat our mortgage rate with savings - we're currently paying a variable rate of 1.6% so if it goes up again, I'm just going to pay most or all of off with the savings we have - in hindsight we maybe should have fixed a few months ago but because we only owe around £25k, we would have needed a zero fee deal and the rates weren't that great, plus I knew we could just pay it off, but in reality I'm probably going to hedge a little and split the savings between mortgage overpayment, investing and keep some back in cash partly as an easily accessible emergency fund and partly because we could probably do with new, less drafty windows now that heating has become stupidly expensive.
So in short, despite normal MN wisdom to aggressively overpay your mortgage, for the last 10/15 years it has rarely made sense to do so, and for anyone who's currently on such an excellent fixed rate that you have, you're throwing money and financial good sense/luck away for the enjoyment you'd get from seeing you mortgage balance go down.