Finnair has some system where you can bid for upgrades which ought to give a much lower price than the £800 recently quoted for BA LHR-PVG on the AUP thread. I've never tried it (maybe there's some experience over on the Finnair board). I always go straight for business as you can often get a J fare to China for the price of BA WT+.

You will loose a bit of time with the transfer at HEL, but if you have access to the lounge it is a pretty pleasant place to while away a couple of hours. And if you are in Y the stopover will at least give you the chance to stretch your legs.