It's up for debate.
I currently do a cash league with mates where the winner gets a huge chunk of the admission and the prize gets smaller where the last place player pays off a big amount. I can't see that working for an online league though.
I suggest an entrance fee sent to an admin at OCUK and then the top 3 share the prize pool.
For example, 20 players want to join. 10GBP entrance fee. That's a 200 quid kitty. First place would get 100 quid, 2nd gets 75 quid and 3rd gets 50 quid. It can all be done using Paypal.