Only citizen of a specific country can represent it in International Sport Competitions.
Also according to the International conventions (for example, European Convention on Nationality) there are three kinds of legal status in citizenship: citizen, non-nationals, multiple nationality.
If a person has the nationality of two or more states, he must choose a country, which he is going to represent the international competitions.
Stateless persons may represent the country in international competitions only after the adoption of the citizenship of a specific country.
Photo's of the British Good ones are from Robin Hyslop and Steve Jacob