If you have automatic membership renewal enabled then disable it or remove your bank card details from your account. Otherwise you don't need to do anything other than maybe use up any outstanding membership allowances, cash credit etc. Then you can just contact another provider and join them and ask them to port your number from 48 if you want.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.