For LON-HEL try
www.blue1.fi or
www.easyjet.com, for Tampere, which is 160 km north of HEL you can fly
www.ryanair.com.
Between Stockholm and HEL you can take a ferry by
www.vikingline.fi or
www.siljaline.fi Book a cruise (return trip) and skip the return portion of your ticket, itīs often cheaper than a oneway.