Would you consider an additional "Koreanised" main?
London-based Korean Youtuber/ Chef Gabie Kook did a nice ssamjang pork roast for a dinner party.
Link to her video here:
It looks easy enough, and ssamjang is readily available at most East Asian supermarkets.
Apart from that, I do agree with PP that you should stick to a traditional British Christmas meal with all the trimmings.
It's what they will expect, and it would be lovely to give them the full experience, so to speak.
Feel free to kick the flavours up a notch, by all means - the most consumed vegetable in South Korea by far, is chillies, after all!
Make sure veggies are well-seasoned, (bacon is big in South Korea, so consider sauteing your sprouts with that?), and offer loads of punchy chutneys/ preserves/ sauces.
Lastly, fruit is seen as a luxury item in South Korea.
It's a popular gift, and can be very pricey, hence the "prestige".
Perhaps consider adding a fruit-based dessert (meringue in shape of Christmas wreath with berry coulis?), in addition to your usual pud.
And also yes to PP who suggested offering them winter slippers.
Wearing "outside shoes" into someone's home just isn't done in South Korea.
It's seen as rank.
It would be a nice gesture on your part, and I'm sure SIL would appreciate you making his family as welcome as possible.