But the OPs mortgage rate is currently 1.42%. She can get more interest in an interest access savings account. Or put it in a fixed savings product for a little more, then possibly apply it to the mortgage at the end of the fixed rate period.
OP, in your situation, the only reason to overpay your mortgage would be to get a better LTV on remortgaging, although I'd do that at the end of your current deal, not now, or if you'd otherwise spend the money on shite and end up with a bigger mortgage and no savings. Or possibly if there's a chance you'd have to claim universal credit, as in that case, you'd be expected to live on the money until it reduces to under £16k. Otherwise, when people say 'no point overpaying, the interest rate is so low' they are right.
As for what I'd do with the money, in your circumstances, I'd think about when I might need/want to use it. If some years away, invest some or all of it in an ISA or pension. But not if there's a chance you might want to upsize in the short to medium term.
In that case, you need to save, not invest, and then I'd be looking at a fixed rate savings account, or even premium bonds, you should get a similar payout to your mortgage interest rate, although this is a gamble.
The other thing I'd think about would be whether it's worth using some of the money on energy saving measures that might otherwise be unaffordable, eg better windows, boiler, insulation, solar panels, or even an electric car.