I'm surprised Finnair is expensive.

Looking for a random date in November, so quite soon, Finnair is competitive with all the other one-stop options that don't require an overnight transfer (LOT, which has a 14h layover, looks pretty good given that WAW is a nice airport and the timings are convenient if you don't mind sleeping in an airport hotel mid-trip).

Going all the way forward to June, Finnair is again competitive on a one-way (LOT is again cheaper, but otherwise you'd have to deal with something like Air Astana).

It's usually a little cheaper to fly Finnair from an EU starting point, but you only get 360 return TPs for that routing (albeit plus positioning flights) whereas UK departures get 440 return. Once you account for APD and Heathrow charges, the difference is minimal and barely worth the cost of a positioning flight, let alone the time, effort, and risk.

If you want to go via Qatar, bear in mind that it's in no way en route between northern Europe and Korea - Finnair is a better, shorter routing as well.