Onlyfans dosnt charge a fee when they pay you (At least I have never had a fee here in europe or heard of anyone getting charged a fee to withdraw). Any fee's you encounter during a withdrawal are down to the payment method you use. The fee's for both paxum and cosmo can be found on their websites, and if you cash out to a UK bank account then you wont have any control over the exchange rate being used (which are generally pretty bad) and again any fee's are going to be dependent on the bank that you use.
Paxum currently has a fee of $0.25 cents for every deposit and a $0.25 cent fee for loading the pre-paid card so to ave on fee's wait until you have finished receiving all your site payments before loading the card. Withdrawing from paxum via bank transfer can be expencive (cant remember the fee, just remember looking at it as an option when there was issues with their old card issuer and thought it was on the high side).
So if onlyfans is the only site you are using to get paid, you are looking at $0.50 in fee's, which imo it's more than acceptable for instant payments (they usually arrive in under 2 mins to the paxum account) vs the 3-5 business days of a regular bank transfer.