Bus journey Dover (UK) to Stockholm costs £100 single one way by Eurobus , change Brussels, Hamburg, and Copenhagen, same trip is available from London, connecting times are quite tight, then ferry to Heslinki or Turku, which you need, as sleep is almost impossible on coach time span,( can only relate to actual journey taken last May).
Left Dover 14.00 Thursday arrived Turku 20.00 Saturday.
As Hank says fly, take Ryanair from Stanstead ( around £ 39 all in) arrive 2 1/2 hrs later at Tampere 21.30 , connecting coach direct from aiport to Helsinki ( €20), arrive before midnight.
Note Ryanair fares are special offers which require pre booking.
As for the bus trip, at least you get to see France, Belgium, Holland,German ( watch out for their customs officers

, Denmark, Sweden and of course one of the most enjoyable ferry trips you'll ever experience. Your choice

The only difference between a rut and a grave is the depth.