As I understood, Crowdcube only charges the funding company, I think it is along the lines of 7% per pitch, then Stripe charges them for the payment processing on the top.
But investors do not suffer in any form: if you invested £10, you will get shares rounded to £10, whereas Seedrs only allow the increments of share’s value (if a price is £7.50 per share, you could only pay whatever is divisible by £7.50 with no decimal places)