Parents have a duty to maintain their children irrespective of the amount of time they spend with them or what country they live in. However international payment can be difficult to enforce. Parents cannot enforce an arrangement made informally between them, it must be made legally binding first.
Reciprocal Enforcement of Maintenance Orders (REMOs) are international agreements between countries which can help parents in the UK recover child maintenance when the other party is living abroad and vice versa. 101 countries currently have an REMO agreement with the UK
If you make a REMO application it will be enforced according to the laws of the country where the paying parent lives. They are not necessary if the paying parent works for a British organisation abroad, as the UK still has authority over these companies and maintenance can be enforced in the UK.