Both are FCA regulated and FSCS protected. The real differences are fees, investment range and how each platform feels to use. Here is the honest comparison.
Fees verified July 2026. Capital at risk. Information, not financial advice.
Portfolios above roughly £60k where a flat fee beats percentage charges.
Index fund investors who want the lowest possible total cost and are comfortable with a newer app.
| Fee | Interactive Investor | Prosper |
|---|---|---|
| Platform fee | Core £5.99/month (up to £100k); Plus £14.99/month (no limit); Premium £39.99/month | £0 |
| Share dealing | £3.99 per trade (£2.99 on Premium) | Not applicable (funds and ETFs) |
| Fund dealing | £3.99 per trade (£2.99 on Premium) | Free |
| FX fee | 1.5% on the first £25k, tiered lower above | None on GBP fund classes |
| Stocks & Shares ISA | Included in the monthly plan | Free |
| SIPP | Included in the monthly plan | Free |
| Withdrawals | Free | Free |
| Minimum to start | No minimum (£25/month for regular investing) | No minimum |
Long-standing customers value the flat fee and the breadth of investments.
The 1.5% headline FX fee and occasional platform outages are the recurring complaints.
Read Interactive Investor reviews on TrustpilotEarly adopters praise the zero fees and responsive founding team; roughly 84% of reviews are five stars.
The review base is small and some users want more account types and a web version.
Read Prosper reviews on TrustpilotInteractive Investor charges a flat monthly subscription instead of a percentage. On a £200,000 portfolio, £5.99 a month is a fraction of what percentage-fee platforms charge, which is why ii keeps winning larger DIY investors.
The equation flips for small pots: £71.88 a year on £10,000 is 0.7%, more than almost any rival. Work out your portfolio size first, then decide.
Prosper's pitch is the cheapest total cost of ownership in the UK: no platform fee, no dealing fees, and refunded fund fees on a list of mainstream index funds. For a straightforward global tracker in an ISA or SIPP, the all-in cost can genuinely be zero.
The counterweight is maturity. It is a young platform with a small (if very positive) review base and no individual shares. If that trade-off suits you, the price is unbeatable.
A 0.2% fee difference is worth optimising. Knowing whether you are saving enough in the first place is worth far more. Delphina models your pensions, ISAs and investments and tells you where you actually stand.