If you put it in pension now, you might get some tax rebate on it, but it will grow a lot during the time it's in your pension fund and you'll only be able to take 25% of it tax free. You will be taxed on the remaining 75% ( which will be a lot more than the tax rebate you get when youput it into the pension)
Thinking about it, I can understand why you've made this mistake. It's important though, so I'll give a worked example.
Say you inherited £40k and were deciding whether to put it in ISA or SIPP.
If you put it in the ISA, the ISA will have £40k. If the ISA value doubles by retirement, you'll have £80k which you can take out at any time. There's no tax on the growth, and no tax to pay on withdrawal. You get 80k
If instead you put £4k from your salary into your pension each year for 10 years, getting the tax relief. Without growth, your pension would hold £50k because of the 20% tax relief. In the same funds, it would likewise double by retirement to hold £100k (minus the trading fees to move from ISA to pension which won't be significant). As you point out, you don't pay tax on the growth, but you do pay income tax when you take it out. At 20% (if withdrawn over a number of years - staying in basic rate tax band) you would expect £20k tax due which would give you £80k - same as with an ISA, because it's the same percentage. But actually, we get 25% tax free, up to a cap (which the OP probably won't reach). So with that, you get £25k tax free, and pay 20% on the other £75k. So you only pay £15k tax, and you get £85k
(Plus the extra money her employer put into the pension scheme over the 10 years, now that she has opted in. Maybe £5k extra for that, at £15k/year salary.)
Now, it's a bit more complicated because it all depends on when you take it out of your pension, because of different tax bands.
If OP wanted to take it all out in one go to pay for something big whilst she was still working, she'd still get the £20k tax free lump sum from her pension, but she'd pay income tax on the remaining £80k + her £15k salary. That would take her well into 40% tax, and the extra £25k tax she would have to pay (yay for progressive taxation!) is more than her £20k tax free lump sum. She would only get £75k
On the other hand, if she stops working at 62, then not only does she get the £20k lump sum, but by drawing down the £80k over 5 years (and putting it back into an ISA at that point if she doesn't need it immediately), her income each year is £16k so tax is only £700 because of her personal allowance. In total, she would pay only £3500 tax, and would get £96.5k
The important thing is to think about your expected cash flows, and your different tax rates and allowances at different times of your life. Try out different options and see how it works out!