all you need to do is log into each account and switch OFF recur / auto top up here https://giffgaff.com/profile/payment-details
and remove card details
balances can not be refunded, so perhaps switch OFF recur / auto top up and use up any remaining balances first?
if you wish to keep your phone number request a PAC and give this to your new provider, simply dial 43431 for your PAC number
you don't need to give any notice if you decide to leave
your account will remain open if you decide to come back and if you don't wish to keep the phone number your gg sim will be automatically deactivated after 6 months non use and the phone number is recycled, if you decide to transfer your number away from gg your sim is deactivated when number has been transferred