Helsinki has more lounges than is sensible in such a tiny space (one in Domestic, at least 2 in Shengen area that I recall and one in International)! I assume you mean the international lounge before your return flight? If so, it's pretty decent - after passport control, by the gates you'll depart from. HEL is nice and compact and simple anyway. A few snacks and the usual selection of booze to be expected - including beer on tap ^ Fairly bright/clean/Scandinavian 'feel' to it. The lounge alone isn't worth an upgrade price IMHO but the J flight (~3hr block time) and lounge together are if you book on to the AY operated ones (better service offering than CE). I class return on this route now runs to 409.60 on ba.com with CC surcharge (thanks to El Gordo and his taxes that is) but the lowest Y price can often be much less. Depends when you book and how much the difference is...do try to book under the BA codes on ba.com since (a) it's normally cheaper and (b) you'll benefit from the higher cabin bonus under the BA code.

Just try to avoid the A320 services if you can where AY have gone to 3-3 in business with no guarnatee of middle seat blocking I have my first one of those coming up in a couple of weeks.