it is usually based on the country of citizenship of the applicant or if the applicant has 12+ months of temporary status in another country. Also, some visa offices are used to help with file overload, so sometimes a file will be sent somewhere else to help reduce backlog.
Where is the applicant currently living?
Where does it say the application is being processed in London? (If it was the SA letter, double check it is referring to the correct applicant.)
