I'd definitely have a floor lamp to the right of the window 
And another somewhere around the back of the armchair/unit.
I think the dark furniture is sucking a lot of light out of the room but that's difficult to change. Add lots of light cushions? I had to trial and error with a selection from somewhere like Dunelm as I never get it right first time! I currently have five on my sofa (two larger 50cm ones and three 40cm) and two 40cm on the armchair. Your sofa is bigger than mine.
I've just added a big (pale) neutral rug from La Redoute (with a zigzaggy/Berber design) to tie my seating together. Not sure if you have space for a small coffee table? Maybe something with glass to reflect the light?
I was going to suggest mirrors to bounce the light around. I see you have one over the mantel. I'd be tempted to add a medium/large one over the back of the sofa to make the most of the light from the window. Move the current frames along the wall or into one of the alcoves.
Is your redecorating including painting? I'm not crazy about the buttermilk colour with the furniture/carpet. But that could just be me! I prefer blues/greens/pale greige/taupes with such dark brown furniture.
I'm still finding my way with ceiling shades. I like quite large shades for impact but then I don't really use them as a light source, mainly decorative 
Is it a long thin room? They're tricky to furnish. I was trying to figure out if you could swap your furniture around to have the sofa as a room divider where the armchair is (L shaped bit against the right hand wall and walkway/entrance by the fireplace) and the chair over by the window. But I don't know if the sofa's too wide.