The foreign bank branches in Korea are for providing their services to their corporate customers in Korea. Most of them don't offer the retail banking services in Korea and a few branches offer limited retail banking services.
Also, the foreign bank branches in Korea don't share the customer and account information with the branches in their home country. Hence, even if you have an account in your home country, they can't provide the banking services with it in Korea.
If you need to withdraw money from your bank account in your home country, you need to issue an ATM card from the bank in your home country and use it at a Global ATM in a Korean bank branch.
For more information about banking services, contact the bank branch.
HSBC(Hongkong and Shanghai Banking Corporation) is a multinational bank based in the UK.
They used to offer consumer banking business in Korea. But they announced the discontinuation of their consumer banking business on 8th July 2013, because they couldn't make enough income in the consumer banking business. They had 11 branches around the country, but they shut down all of them except one on 21st March 2014. The existing account holders can still keep and use their account in HSBC Seoul branch.