Finnair is
specialised in flights between Europe and Asia. They follow the
northern route to Asia, which is the shortest way.
There seems to be a daily flight leaving Bangkok at 00:30 past midnight
and arriving in Helsinki at 6:30 am the same day in 2009. It's flight
AY 096.
There also seems to be a daytime flight 5 days a week (Friday till
Tuesday in november 2009) leaving Bangkok at 9:25 am and arriving in
Helsinki at 3:15 pm. It's flight AY 090.
They sell a return ticket departing in Brussels (BRU) booked in
november 2009 for a flight via
Helsinki (HEL) to
Bangkok (BKK) and back in May 2010
for around 700 euro.
One way seems almost the same price.