giffgaff are the company that handle your phone calls. If you move abroad you can change your phone call package to just Pay As You Go (no monthly goodybag), so if you don’t use it then you don’t pay for any calls.
But you also seem to say you bought a mobile phone with a loan through RateSetter (the loan company). If you signed a loan contract then you still have to pay for the phone you were given, until you have paid for the loan in full. You can overpay and pay it off early if you want (it saves you interest so you pay less overall).
If you are leaving the country and do not need the phone anymore then it may be worth considering selling the phone and then using that money to pay the loan off early.
If you have issues repaying a loan then as it is a legal agreement you should contact someone for independent financial advice. If you just temporarily need to change one payment then the loan company may agree to it if you ask them, but seek advice if it is longterm as it will affect your creditscore and your ability to get any finance, creditcards, loans or even buy a house in the future.