You're right in what you're saying where tax at source or tax at the end end up in the same numbers you get in your pocket at the end. But you're still getting the 25% lump back tax free etc, that's a massive difference if people are fine waiting for it. Paying extra for the ISA route is the penalty of freedom I suppose.
But the main point is the guy paying 20% tax, deferring 20% tax isn't going to be getting the same benefit as the guy paying 40% tax, deferring 40% tax, as they both likely take it back out at 20% or less. It's a much bigger winner for the rich (like a 20% discount), which is why **** loads of rich people are doing it, or should be doing it, and they can afford to do it to (which the 20% guy probably can't as much).
I don't want to confuse things with public sector/ defined benefit, as I'm not in that boat, other than my old forces pension which I'm just leaving alone.
I'm gonna work this out more thoroughly for my own circumstances, as it's probably going to make more sense me putting more in a pension once I've got enough in an ISA nest egg to cover the house and to to cover the gap between potential retirement and to when a pension could be accessible.