British Airways is offering direct flights from London Heathrow to Seoul for 499 GBP, round trip including taxes.
The travel period for this fare is January to March 2013. Book by 22nd January 2013.
Use the British Airways 'Low-Price finder' to find the cheapest fare. Premium Economy seats are available for 747 GBP, return incl. taxes.
If you're looking for a cheap accommodation in Seoul, check the current 40% + 10% Hotels.com sale and voucher but always compare prices on HotelsCombined.com.