Your application WILL be processed in Seoul if you have been "lawfully admitted for a period of at least one year". If you prove that you have been admitted for one year in Korea (proof of status in country and when it began), it will be processed in Seoul which makes it easier to communicate with them quickly when they request more information, or possibly an interview. If you haven't been there for a year, or are not going to be there the whole time the application is processed, it will be done in London.
Another thing- since your partner is a Canadian living exclusively outside Canada at that time they will sign the undertaking, you have to prove that you and your partner intend on settling in Canada when the application is finished processing. The proof you need to provide for that is listed here in the guide, on page 7.
http://www.cic.gc.ca/english/pdf/kits/guides/3900E.pdf