I too am confused about this.
From the annual costs and charges Statement I copied the following:
"A fee charged if you hold investments on a share dealing
account or ISA at the end of a quarter, and have traded
fewer than three times in that quarter. The fee does not
apply if you only hold cash on your account, and any
commission you pay reduces the fee by the relevant
amount."
From this it seems that if you have traded three times irrespective of whether it is in US or UK markets you should not be charged but I'd call IG just to be sure.
From where did you find the £96 fee charge?