Apple care plan monthly payments

If you're talking about Apple Care Plus, you must subscribe within 60 days from the date of purchase.

And you can do it online too:--> AppleCare Products - iPhone - Apple (UK)

Regards

Giulio

I'm unsure whether monthly payments for AppleCare are available in all countries, but check https://mysupport.apple.com/add-coverage/ to see if there is an option there.