Capital gain tax on forex

I have trade in a forex trading platform (uk) called forexf.cc. I can’t withdraw my money if I don’t pay the capital gain tax on the platform. Do I have to pay those taxes on the trading platform or I ll have to pay taxes in France or both? Why they can’t deduct the taxes from my gain and capital?

Hi Vane Lecuyer Ortiz,
For tax purposes, the activity of buying and selling securities generally falls short of trading and is subject to Capital Gains Tax (CGT)
Please note, however, that CGT is not payable up front, so any platform fees you are  being charged should be taken up with the platform.
Please also note that capital gains on securities should be declared in your country of residence.  
Capital Gains Tax: what you pay it on, rates and allowances
Thank you.