Coming from London, Seoul doesn't seem overly expensive, but it is a big, vibrant city so you can't expect it to be cheap.
Accommodation will probably be your most significant expense. Transportation is pretty affordable, and you can eat quite cheap in Seoul if you eat around the markets and small local restaurants.
However, you will find a great variety of foods and desserts in Seoul which makes it very difficult to keep in the budget. There is always something else you just have to try right now.
1 GBP = KRW 1464
1 USD = KRW 1132
1 EUR = KRW 1348
1 CNY = KRW 174
Street market food:
Meal for 2 in local restaurant:
Bottle of water (0.5l):
on average £39/night
from KRW 1250 (one journey with T-Money card, inside the city center you won't pay much more)
from KRW 3000 (better for shorter distances, 1-2 subway stops, and 2+ people)
cost nothing, just a great experience
For detailed information read more in the transport section.
Tipping is uncommon in Seoul and generally not expected.
ATMs are widely available.
Cards (VISA, MasterCard, Amex): Depends on where do you mostly spend your time. In shopping centers, bigger restaurants and chains you will be ok with all major cards.
In small local shops, restaurants, and markets you need cash.
In taxis, I usually pay by cash or use T-Money card.