You can't beat white for brightness. TBH I don't know if high-sheen makes much of a difference but do make sure you get Brilliant White rather than normal White - it's a much brighter, warmer colour than normal white which is distinctly grey by comparison. (I have this in both north and south facing rooms in my house and recommend it to everybody - Woodie's own brand)
Also maximise the light that is coming through your window by removing any blinds or curtains (if privacy is an issue I would recommend net curtains as you lose minimal light with them and they diffuse any light nicely - assuming you keep them clean!).
Another trick I have seen (though have not used myself) is to put mirrors in the recess on both sides of the window to visually extend the width of the window and bounce more light into the room. Any glazier will cut the mirrors for you and advise you how best to attach them to the wall.
Pictures in frames will also help as the shine of the glass will reflect light around too.
If possible, it will make a huge difference if can replace or paint your cabinets, especially the uppers, with light colours.
And, as mentioned, your lighting will have a big impact too. If you put spotlights on the ceiling (you can get fittings that have several spotlights in one fitting) and then, most important, point them up at the ceiling rather than down at the room. This will bounce the light into the room and give it much softer light as well as much softer shadows from the edges of cabinets etc. Ikea also do under-counter lighting that you can just plug into a socket so you don't need to worry about rewiring.