The lounge situation is quite complicated in HEL. There's also Gateway Fresh Lounge on the non-Schengen side, but I don't know if that's accessible to you. If you're on AY flight, you might get in as OW Emerald; if you're on BA flight, tough luck. Things might have changed but I wouldn't hold my breath.

I would call Finnair and ask them which lounges are open and which ones you can use. Gateway Lounge is closed on weekends so Silver Wings would be your only choice if you can't get in Gateway Fresh Lounge.

You can read about my adventures with the Gateway Fresh people here:
http://www.flyertalk.com/forum/showthread.php?t=530321