Your Cash ISA Only Holds £20,000. Premium Bonds at 4.35% Tax-Free Beat a Taxed Account on the Rest.

The £20,000 ISA allowance is the most valuable tax shelter most savers will ever get. Fill it first with a best-buy cash ISA at 4.61% easy access or up to 4.87% fixed. That produces £922–£974 a year on a full £20,000 with zero tax and zero risk. The allowance is set out on GOV.UK.

The allowance is also finite. £20,000 a year sounds generous until you sell a property, inherit money, or let a bonus land. The cash above that line faces a 45% marginal rate for additional-rate taxpayers and no savings allowance at all.

That is where the Premium Bonds decision actually lives — not 'instead of an ISA' but 'after the ISA is full.'

On £50,000, the 4.5% taxable account pays £2,250 gross. After tax that is £2,000 for a basic-rate saver, £1,550 for higher-rate, and £1,237.50 for additional-rate. Premium Bonds' expected £2,175 is tax-free at every band — ahead of the taxable account by £175, £625 and £937.50 respectively.

An interest rate only matters after tax, and tax is where the comparison tilts.

A 4.5% savings account looks like it beats Premium Bonds' 4.35% by 0.15 points before tax. But the Personal Savings Allowance is £1,000 for basic-rate taxpayers, £500 for higher-rate, and £0 for additional-rate. Above it, every pound of interest is taxed at your marginal rate — 20%, 40% or 45% depending on your income tax band.

For an additional-rate payer, a 4.5% account is really a 2.475% account after tax. Premium Bonds' 4.35% — mean, tax-free — wins by a wide margin once the ISA and allowance are used.

The basic-rate saver nets £2,000 from the taxable account against £2,175 expected from Premium Bonds — ahead on expectation, but with a caveat: £2,175 is a mean, not a promise. For higher and additional-rate payers the margin is large enough that the 'expected vs guaranteed' trade-off stops being a tie.

Assume a saver who has already used the ISA. The next £30,000 has three destinations: a 4.5% taxable account, a 4.82% one-year fixed bond, or Premium Bonds.

The fixed bond's headline rate is the highest, but tax decides who wins. On £30,000 at 4.82%, the gross interest is £1,446. After the Personal Savings Allowance, a basic-rate saver nets £1,356.80 — the fixed bond wins. A higher-rate saver nets £1,067.60 and an additional-rate saver £795.30, both below Premium Bonds' £1,305 expected. The fixed bond beats Premium Bonds only for basic-rate payers once tax is applied.

Premium Bonds' edge is the combination: tax-free, accessible within days, and Treasury-backed. A one-year fix pays more but removes access; an easy-access account pays 4.5% but hands 40–45% of the interest to HMRC above the allowance. For higher and additional-rate payers, Premium Bonds are the only option that is simultaneously tax-free, instant-access and government-backed.

Our complete Premium Bonds guide runs the full NS&I rate card and the after-tax breakeven at every balance.

Premium Bonds carry HM Treasury's 100% guarantee with no upper limit. Every high-street bank and building society, by contrast, is protected by the FSCS only up to £120,000 per person per institution.

If you are holding £200,000 from a sale, you either split it across two banking groups or you accept £80,000 of unprotected exposure. The same £200,000 in NS&I is backed in full.

The FSCS deposit guide explains the £120,000 limit and how to stay inside it. For cash above that line, Premium Bonds and other NS&I products are the only zero-risk, tax-efficient answer without a second banking relationship.

Be clear about what this argument does and does not claim.

For the first £20,000, the cash ISA wins. Always. A guaranteed 4.61% beats an expected 4.35% — and the ISA shelters it for life, not just this year.

For basic-rate taxpayers with a full £1,000 savings allowance and modest pots, a 4.5% taxable account is effectively 4.0% after tax on £50,000, within touching distance of Premium Bonds' expected 4.35% — and it is guaranteed. The basic-rate saver who needs certainty should not chase the draw.

For anyone who needs a predictable monthly income, Income Bonds at 3.75% AER or a fixed bond beats a prize that might pay nothing for months.

Premium Bonds' case is narrow and real: cash above the ISA allowance, a 40% or 45% marginal rate, and a tolerance for a variable outcome. If that is you, the September rise made a good argument better.

The Bank of England base rate sits at 3.75%, unchanged since December 2025 after a long descent from 5.25% in August 2023. Premium Bonds' 4.35% now stands 0.60 points above it.

The question is what happens next. If the Bank cuts, savings rates fall and the prize fund rate tends to follow — but the tax argument does not change. A taxable account at 4% still nets 2.2% for an additional-rate payer, while Premium Bonds' tax-free status preserves whatever the headline becomes.

CPI inflation at 2.9% is the other half of the picture. A 4.5% taxable account nets a real loss for additional-rate payers once inflation is stripped out; Premium Bonds' 4.35% expected return buys roughly 1.45% of real purchasing power. For the cash above the ISA line, tax efficiency is the difference between staying ahead of inflation and drifting behind it.