Max gains

Total return, assumes reinvesting; but this doesn't account for regular (monthly saving) additional purchases. Max return possible since the bottom out when brexit referendum was announced, up to today is 35.30%, 15% of that is 5%, but given that you did not by everything on the night of brexit announcement, actual net gains should be significantly less than 35.30%, thus effective amount to pay in capital gains tax should be something like 2-3% I think.