Not sure if you can do an order with your entire free funds but anyway what happens isā¦
You make an order to buy Ā£1000 worth you will be able to buy slightly less shares than without the FX fee because your currency conversion between Ā£ and $ will not be at the spot rate as it is now, it will have the 0.15% fee applied
So there is no further deduction on top of the Ā£1000, your Ā£1000 just buys you slightly less value of $$$ shares than if there was not a fee
Check the article on the help centre for more info