1
InvestEngineBest pick
0% platform fee across the ISA, SIPP and GIA, with 0% dealing fees on DIY portfolios — the lowest total cost of these four, provided an ETF-only range works for you.
2
Trading 212
0% platform fee and 0% commission on the ISA, matching InvestEngine on the headline cost — the 0.15% FX fee only applies if you hold non-GBP assets like US stocks.
3
interactive investor
The flat monthly fee isn't free, but it doesn't scale with your portfolio — for a large ISA or SIPP, it works out cheaper than a percentage-based platform fee like Hargreaves Lansdown's.
How we chose
We compared platform fees, dealing fees and FX fees together, with a UK-specific note: cost that scales with your portfolio (like Hargreaves Lansdown's percentage-based platform fee) behaves very differently at £5,000 versus £500,000. Hargreaves Lansdown falls outside the podium in this cost-focused ranking despite cutting its fees in March 2026, because a percentage-based fee is structurally more expensive at scale than the 0%-fee or flat-fee alternatives here — the full review explains where it still makes sense.
Frequently asked questions
Is the broker with the lowest order commission always the cheapest?
No — you also need to add the FX fee and any custody or inactivity fee. A broker with free orders but a high FX fee can cost more over time than one with a small fixed commission but no other costs.
Is it worth choosing purely on lowest cost?
Cost matters but isn't the only factor — market coverage, support in your own language and deposit protection matter just as much, especially for those without prior online investing experience.