I am no expert but I'd not see an ISA investment as a substitute for a pension, more as something you do in addition, if you can afford to. Do you have an employer who offers a pension scheme ? In the past those have been hard to beat; but of course they are all substituting "guaranteed" payouts for "guaranteed" contributions these days, so they may not be any better than doing your own pension scheme search.
Not that any provision for older age is perfect. You do what you think best and society has a way of ensuring it wasn't as wonderful as you thought. I reckon I should have bought and buried gold bars in the garden.
(Anyway most self provisioning these days results in being ineligible for stuff those who couldn't or didn't bother to self provide, are offered.)