Costs don't matter in the abstract — they matter compared to what each trade actually makes. A £10 cost is nothing on a trade that nets £200, and fatal on one that nets £12. Set your typical trade and see how big a bite friction really takes.
On an average trade, roughly…
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How to read it: everything here hangs off one ratio — your cost per trade versus your profit per trade. That's why the same £14 cost is trivial for a patient, big-target trader and ruinous for a scalper chasing tiny moves. (This is a simple per-trade view; on a compounding account the gap grows wider still over time, because money paid in costs never gets to grow.) A teaching model on steady averages — not a promise about any real strategy. Not advice.