The pension/ISA conundrum depends on the gap between the expense ratios on each option and the timespan.
Option A: Contribute £1,000 post-tax into S&S ISA, 0.5% TER (total expense ratio)
Option B: Contribute £1,320 pre-tax into pension, 1% TER
If both options are generating a 5% real return before fees and your contributions remain the same, then let's say you pay in until you FIRE at 45, and they both carry on compounding until you reach 55. In that case by my calculations you're looking at £394,833.91 (A) versus £474,327.69 (B). On face value that looks pretty much even (factoring in 20% income tax on withdrawals from option B), but remember you can now take a 25% lump sum of your pension tax free. So if you did that, and salted the tax-free lump into your ISA over a few years, option B would be a clear winner.
Of course if the gap in TER is higher, or the pension fund underperforms your ISA, or you retire at 55 rather than 45, or you move into the 40% bracket, then the results change. The rules around pensions and ISAs can and probably will change over that time period.
Don't trust my numbers, but do run your own. I was, like you, initially all gung ho for ISAs over pensions (lower expenses, not locked until a certain age), but in my case contributing more to my pension makes sense, and it might do for you too.