Perhaps I haven’t explained myself well enough.
Firstly, let me make it explicit - I am not saying cheat HMRC. That is a very bad idea!
What is possible is to add funds to an ISA, to have the fees taken from your contributions before they enter the ISA wrapper, and to then have the net amount enter the ISA wrapper. You send your max contribution + the fees, the fees are taken, the max contribution enters the ISA wrapper.
Does that make more sense to you?