immigration will see it as round trip as you are not going to third country in transit
however: Visa required, except for Nationals of China (People's Rep.)

with a visa issued by Australia, Canada, New Zealand,
Switzerland, USA or an EEA Member State and

Visa required, except for Passengers with a British passport

with nationality of British Citizen or British National

(Overseas)" shown on the bio-data page for a maximum stay of
90 days.
better check to make sure but it looks like that if they have a visa for the u.s. then they're visa exempt in korea.