You'll have to call Apple to cancel apple care: https://support.apple.com/en-gb/HT202704
you'll get a pro rata refund from apple from the day you cancel, not the month.. the longer you keep it, the less you get back. I wouldn't expect that different to be much though.. a few quid maybe.
to pay off Barclays you need to call them (have your agreement number ready): 0800 15 22 888
Just to add, Bare in mind any refund off AC+ goes direct to Barclays