Is an ISA actually worth it for me?

Everyone gets a Personal Savings Allowance: £1,000 of interest tax-free at basic rate, £500 at higher rate, and nothing at additional rate. At 4.5%, £1,000 of interest takes roughly £22,000 of savings to earn.

So below that, a basic-rate taxpayer pays no tax on savings interest anyway — and an ISA that pays a lower rate than a normal account actively costs you money. Chasing the wrapper instead of the rate is the most common mistake in UK saving.

It flips as your balance grows, if you move into higher rate, or if you are additional rate and have no allowance at all. And a stocks and shares ISA is a different argument entirely, because capital gains and dividends are not covered by the PSA.