A platform fee is what a broker or platform charges to hold your investments, separate from any fund's own charges. It comes in two shapes. A percentage fee is a share of what you hold, so it grows as your pot grows. A flat fee is a fixed amount whatever the pot's size: £120 a year is 1.2% of £10,000 and 0.12% of £100,000. Some platforms put a fee cap on their percentage fee, often only for shares, ETFs and investment trusts rather than funds.
Over ten years, a percentage fee costs more than the fees themselves. Take £20,000 growing at an illustrative 5% a year: with no fee it reaches £32,578, and with a 0.45% yearly fee it reaches £31,141. The fees add up to £1,163; the rest of the £1,437 gap is growth that money would have earned had it stayed invested. The app doesn't record platform fees. If yours is paid by selling part of your holdings, record those sales on the holding, or the app will show shares you no longer own.
Illustrative: a steady 5%, fee taken once a year. Real returns are uneven; the fee is charged either way.
Educational content only. It is not personal financial advice.