Three-quarters of all ISA money in the UK is sitting in cash accounts. Which means three-quarters of ISA holders are doing something that feels prudent but isn’t.

This isn’t a criticism of the people who made that choice. Nobody told them otherwise. The financial industry makes far more money from complexity than it does from telling you to do something simple — and “just invest in a low-cost index fund” is devastatingly simple. So they never tell you that.

A cash ISA paying 4.5% sounds decent. Inflation is running at around 3.5%. Your real return — the actual increase in your purchasing power — is roughly 1%. In a good year.

That’s not safety. That’s just a slow death.

Now compare that to a Stocks & Shares ISA invested in a low-cost S&P 500 ETF. Over the past ten years, the average stocks and shares ISA has returned around 9–10% per year. Not every year — some years more, some years less, occasionally negative. But compounded over time, the gap becomes almost painful to look at.

Here’s a comparison that makes it concrete. Someone who has put their full annual ISA allowance into cash every year since 1999 would have, by early 2026, roughly £470,000. Impressive.

Someone who put the same money into a stocks and shares ISA — using a simple low-cost S&P 500 ETF — would have around £1.58 million. More than three times as much. Same money. Same tax wrapper. Completely different destination.

Yes, a Stocks & Shares ISA can go down. It will go down — probably several times over a long investing life. There will be crashes. There will be headlines. There will be a period where your balance is lower than it was.

A cash ISA doesn’t do that. The number on the screen never falls.

But here’s the thing most people miss: that stability has a price tag. The price is everything above inflation that you didn’t earn. The price is the compounding that never happened. The price is the gap between £470,000 and £1.58 million.

Risk isn’t just the chance your investments fall in value. Risk is also the chance you end up with far less money than you could have had. That risk is invisible in a cash account. It doesn’t show up as a red number. It just quietly happens.

A quick note on what’s changing. The government has signalled that from April 2027, the amount you can put into a cash ISA each year will drop significantly for under-65s. The policy is deliberate: it’s designed to push more money into stocks and shares, where it can actually work for you.

Whether you agree with that policy or not, the direction of travel is clear. The window to park large sums in a cash ISA is closing.

The current tax year — 2026/27 — gives you £20,000 in ISA allowance across all ISA types. The question is simply: what do you do with it?

A cash ISA isn’t always wrong. For money you might need within the next two or three years — an emergency fund, a house deposit, a planned large expense — cash is fine. That’s what it’s for.

But for long-term wealth — money you won’t need for ten years or more — keeping it in cash isn’t conservative. It’s expensive.

The alternative is straightforward. A Stocks & Shares ISA, holding a low-cost S&P 500 ETF, costs almost nothing to run and has historically delivered returns that cash can’t touch. You set it up once, invest regularly, and leave it alone. The less you fiddle with it, the better it tends to do.

You don’t need to be an expert. You don’t need to pick stocks or read financial reports or understand macroeconomics. You just need to stop treating “safety” as the absence of volatility and start treating it as the presence of long-term growth.

Everything I’ve just described — the ISA wrapper, the low-cost ETF, the compounding arithmetic, how to actually buy this stuff — is laid out step by step in my free ebook, Make Yourself Rich.

It’s the complete guide I wish someone had handed me twenty years ago. It’s what I’ve learned over the last twenty years, condensed into a two hour read.

You can get the first few chapters free when you subscribe to Tim’s Wealth Letter. When you’ve read that, and want the rest, you can buy the full book here: Get your copy of Make Yourself Rich here.

The book will show you step by step how to take advantage of the two huge opportunities we now have: tax free investing and low-cost ETFs.

The third magical ingredient is time — the sooner you start, the more you’ll end up with. I’d start now if I were you…

I am not a financial adviser. Nothing here is personal financial advice. Please do your own research before making any investment decisions.