£20,000 is not a coincidence — it's the exact annual ISA allowance, and the alignment between this common lump sum and the threshold is why it's one of the most-searched figures in UK personal finance. If you have £20,000 available before 5 April and haven't used this year's allowance, filling a Stocks and Shares ISA in one transaction is almost always the right first move. Tax-free growth and income, permanently — this allowance cannot be carried forward, so missing the deadline means permanently losing that shelter.

For 40% taxpayers: after filling the ISA, the next contribution into a SIPP is extraordinarily tax-efficient. A £1 net contribution becomes £1.25 via basic-rate top-up from HMRC, then you reclaim another 20p per £1 via self-assessment — a total 67% return before markets move. The SIPP annual allowance is £60,000/year, so most people at this amount have significant headroom. If your employer matches contributions, SIPP comes before ISA every time.

For basic-rate taxpayers, the SIPP advantage shrinks but remains meaningful (25% uplift). The key trade-off at basic rate is access — ISA money is accessible anytime; SIPP money is locked until age 57 (rising to 58 in 2028). If married, your spouse has their own £20,000 ISA allowance — double-stacking before 5 April is one of the most powerful tax moves available to UK couples.