The Cost Stack: What a UK ISA Really Charges in 2026
Most UK investors can quote the headline platform fee on their ISA. Far fewer can say what the account actually cost them last year. That gap matters. In 2026, the difference between a cheap-looking ISA and a genuinely low-cost one is rarely the account charge alone. It is the cost stack beneath it: fund charges, dealing commissions, foreign-exchange spreads, cash drag and the price of leaving.
The headline fee is only the first line
Platforms market an annual charge—0.25%, 0.45%, sometimes a flat £60—because it is simple. But an ISA is not a single product. It is a wrapper around funds, shares, investment trusts and cash. Each layer can carry a cost, and some are deducted inside the fund rather than billed to the account. Two platforms can advertise very different headline fees and still deliver the same net return—or very different ones.
Fair-value rules have made charges more visible, but visibility is not comparability. A percentage fee on a £50,000 ISA is not directly comparable with a flat fee, a tiered fee or a fee that stops at a certain balance. The only useful comparison is the all-in cost of the portfolio you actually hold.
The five layers of ISA cost
1. Platform charge. This is the advertised account fee. It may be a percentage of assets, a flat fee, or a blend. Check whether it applies to cash as well as investments, and whether it is tiered by balance.
2. Fund charges. The ongoing charges figure, or OCF, covers the fund manager's fee and some administration. It does not always capture everything. Portfolio transaction costs and incidental charges can sit outside the headline OCF, particularly in actively managed funds. A 0.75% OCF is not automatically expensive, but it is rarely the whole story.
3. Dealing costs. Buying and selling shares, trusts or ETFs usually carries a commission. Frequent rebalancing, regular contributions and income reinvestment can turn a modest per-trade fee into a meaningful annual cost. Some platforms bundle a number of free trades; others do not.
4. Foreign-exchange spread. Any ISA holding overseas assets—global funds, US shares, emerging-market trusts—will at some point convert sterling. The spread between the rate you see and the rate you receive is a cost. It is often small per trade, but it compounds across contributions, withdrawals and rebalancing.
5. Cash drag and exit costs. Platforms often hold uninvested cash before and after trades. If that cash earns little or nothing while the platform earns interest on it, the investor bears an opportunity cost. Exit fees, transfer fees and per-line charges add a final layer when you switch or consolidate.
A worked example: two ISAs, same £50,000
Consider two investors with £50,000 each in an ISA. Both hold a similar portfolio: 90% invested, 10% in overseas assets, five funds, eight trades a year and 5% held in cash.
Platform A advertises a 0.25% account fee. Platform B advertises 0.45%. On the headline number alone, Platform A looks cheaper. The full cost stack tells a different story.
| Cost layer | Platform A | Platform B |
|---|---|---|
| Platform charge | £125 (0.25%) | £225 (0.45%) |
| Fund OCF | £375 (0.75%) | £100 (0.20%) |
| Dealing costs | £95.60 (8 trades at £11.95) | £0 |
| FX spread | £50 (1.0% on £5,000) | £12.50 (0.25% on £5,000) |
| Cash drag | £37.50 (1.5% on £2,500 cash) | £0 |
| Annual total | £683.10 | £337.50 |
| All-in cost | 1.37% | 0.68% |
Platform B's headline fee is 0.20 percentage points higher, but its all-in cost is roughly 0.69 percentage points lower. That is a difference of £345.60 a year on a £50,000 ISA. If the saving were reinvested and earned 5% annually, it would amount to more than £4,000 over a decade, before any exit fee.
The example is illustrative, not a recommendation. The point is structural: the headline fee is one input, not the answer.
Why the gap is easy to miss
Fund charges are deducted inside the fund, so they never appear as a line on a platform statement. FX spreads are buried in the conversion rate. Cash drag is invisible because it is an opportunity cost rather than a charge. Dealing costs appear only when you trade, and exit fees appear only when you leave. The result is a cost profile that is difficult to reconstruct after the fact.
Investors should also check whether the platform pays interest on cash balances, and at what rate. A platform that pays 0% on idle cash while earning interest on it is effectively charging an additional fee. The same applies to dividend cash, contribution cash and proceeds from sales.
A checklist before switching or consolidating
Before moving an ISA, ask the receiving platform and the existing one for answers in writing.
- What is the all-in annual charge for my exact portfolio, including fund OCFs and any platform fee on cash?
- Are portfolio transaction costs inside my funds included in the quoted OCF, or are they separate?
- What do I pay per trade, per regular contribution and per dividend reinvestment?
- What FX spread applies to overseas holdings, and is it shown as a percentage?
- Does the platform pay interest on cash, and does it charge a platform fee on that cash?
- What are the exit, transfer and account-closure fees, including charges per holding?
- Can my investments transfer in specie, or will they be sold and repurchased?
- Are there rebalancing fees, inactivity fees or charges for holding fractional shares?
The practical conclusion
The cheapest headline ISA is not necessarily the cheapest ISA. A 0.25% account fee can sit on top of a costly fund range, expensive dealing and a wide FX spread. A 0.45% fee can be better value if it buys cheap funds, free trading and a sensible cash policy.
For UK investors, the discipline is to audit the full cost stack once a year, not just at the point of opening an account. Compare the total cost of the portfolio you hold, net of any interest received on cash. Then decide whether the difference is large enough to justify the friction of switching. In 2026, with the annual ISA allowance still a valuable wrapper, the objective is not to find the lowest advertised fee. It is to keep more of the return you have already taken the risk to earn.
