Investment Platforms and Brokers
Cheapest Stocks and Shares ISA: Platform Fees Compared by Portfolio Size
There is no single cheapest stocks and shares ISA, and any article that names one without asking how much you have invested is doing you a disservice. The right answer flips depending on your portfolio size, because UK platforms charge in two fundamentally different ways: a percentage of your pot, or a flat fee that stays the same whatever you hold. Understanding which structure wins at your balance is worth far more than memorising a single “winner”. This guide explains the two models, shows where the crossover falls, and gives representative figures you can sanity-check against each provider’s live pricing.
Fees change often (Interactive Investor overhauled its pricing in early 2026, for example), so treat every number here as illustrative and confirm the current rate on the platform’s own fees page before you commit.
The two fee models
Percentage-fee platforms charge an annual fee as a slice of your holdings, typically somewhere between 0.15% and 0.45%. That is cheap when your pot is small (0.25% of £5,000 is just £12.50 a year) but grows with your wealth. Some cap the fee, especially on shares and ETFs, which limits the damage on larger balances.
Flat-fee platforms charge a fixed pound amount, often a monthly subscription or a per-trade charge, regardless of how much you hold. That is poor value on a tiny pot (a flat fee can swallow a large share of a £2,000 account) but excellent once your balance is big enough that a percentage fee would cost more than the flat amount.
The whole game is working out where those two lines cross for you.
Small portfolios (roughly under £30,000)
For smaller pots, percentage and zero-fee platforms usually win, because a flat monthly subscription is proportionally expensive.
The cheapest option here is often a zero-platform-fee provider. Trading 212 and InvestEngine’s DIY account both advertise a 0% platform fee. The trade-off is that they are ETF and stock focused rather than offering the full range of traditional funds, and they make money in other ways (foreign-exchange charges, spreads or optional extras), so “free” is never quite the whole story. For a simple portfolio of a global tracker ETF, they are hard to beat on cost.
If you want the broader fund universe, a low-percentage platform is the next best thing. Vanguard’s own platform charges around 0.15% (with an annual cap) but only offers Vanguard funds, while other percentage-based platforms give you the whole market at a somewhat higher rate. See our guide to the best investment platforms in the UK for how the ranges differ.
Medium portfolios (roughly £30,000 to £80,000)
This is the band where the decision gets closest, and where the fee cap matters. A low-percentage platform at 0.15% costs £75 a year on £50,000; a flat-fee platform charging the equivalent of £100 to £180 a year is still more expensive at that balance. So percentage models, especially capped ones, tend to stay ahead through the middle of this range.
As you climb towards the top of this band, keep an eye on the crossover. On a 0.15% platform, £80,000 costs £120 a year; on a 0.45% platform it would be £360. That spread is exactly why the fund fee you pay on the investments themselves (the ongoing charges figure) is not the only cost that matters. Our explainer on fund fees and the OCF covers the charges that sit alongside the platform fee.
Large portfolios (roughly £100,000 and above)
Once your pot is large, flat-fee platforms usually take the lead, because a percentage fee keeps rising while a flat fee does not.
Consider the maths. A 0.25% platform costs £250 a year on £100,000 and £500 on £200,000. A flat-fee platform such as Interactive Investor, charging a fixed monthly subscription, stays roughly constant at somewhere around £70 to £180 a year depending on plan. That is a large saving that grows every year your portfolio does. iWeb (now Scottish Widows Share Dealing) takes a similar flat approach with no annual platform fee and a per-trade charge, which suits a large, buy-and-hold portfolio that rarely trades.
Percentage platforms with a shares-and-ETF fee cap (AJ Bell and Hargreaves Lansdown both cap their charge on shares and ETFs) can also become competitive at size, provided you hold ETFs rather than uncapped funds. The key is to check whether the cap applies to what you actually own.
How to find your own cheapest ISA
- Work out your current and expected balance. The right platform for £5,000 is rarely the right one for £150,000.
- Decide what you want to hold. ETF-only zero-fee platforms are cheapest for simple tracker portfolios; if you need traditional funds, you need a platform that offers them.
- Compare percentage versus flat at your balance. Multiply your pot by the percentage fee and compare it with the flat annual cost. Whichever is lower wins today, and remember the flat option looks better every year your pot grows.
- Check the extras. Dealing charges, FX fees on overseas ETFs, and exit or transfer fees can outweigh the headline platform fee, especially if you trade often.
- Re-check yearly. As your balance grows past the crossover point, switching from a percentage to a flat-fee platform can save hundreds a year. Transferring an ISA between providers keeps its tax-free status.
For the ground rules on how ISAs work and the annual allowance, the government-backed MoneyHelper stocks and shares ISA guide is a reliable reference. For how to actually get going, see our guide to how to start investing in the UK.
Frequently asked questions
What is the cheapest stocks and shares ISA in the UK? It depends on your balance. For small ETF portfolios, zero-platform-fee providers like Trading 212 or InvestEngine’s DIY account are usually cheapest. For medium pots, a low-percentage platform such as Vanguard (around 0.15%) is competitive. For large portfolios above roughly £100,000, flat-fee platforms like Interactive Investor or iWeb typically win.
When does a flat-fee ISA become cheaper than a percentage-fee one? When your portfolio grows large enough that the percentage fee would exceed the flat charge. As a rough guide, multiply your balance by the platform’s percentage; once that figure passes the flat platform’s annual cost, the flat option is cheaper. The exact crossover depends on the specific rates, so run the numbers for your own balance.
Are zero-fee ISA platforms really free? Not entirely. Platforms advertising a 0% platform fee still earn money through foreign-exchange charges, spreads or optional paid features, and they are usually limited to ETFs and shares rather than the full fund range. They can genuinely be the cheapest option for a simple portfolio, but read the full charges before assuming there are none.
Can I move my stocks and shares ISA to a cheaper platform? Yes. You can transfer an ISA between providers without losing its tax-free status, which is worth doing once your balance grows past the point where a flat-fee platform beats a percentage-based one. Watch for exit or transfer fees at the platform you are leaving before you switch.
Does the platform fee include the cost of the funds? No. The platform fee is what the provider charges to hold your account. The investments themselves carry their own ongoing charges figure (OCF), which is separate. To know your true cost, add the platform fee and the fund charges together rather than comparing platforms on the headline fee alone.
