When to go
Since there aren't any seasonal elements in the photo, you'll be able to capture a similar perspective of London and the River Thames from Bōkan Roof Terrace (39th Floor) during any month of the year.
I prefer to visit this location in the summer because you can grab a drink, relax and you'll have more time to shoot since the summer months are lighter for longer.
Sunset is the ideal time to photograph the golden, orange and yellow colors coming through the clouds.
It gets really busy during the evening (especially on Friday and Saturday nights), so get there early if you want a good seat.
Field tips
You'll be photographing through the glass, so pick a spot that's clean and free of marks.
If shooting through the glass is difficult, try safely lifting your camera above the glass balustrade, but be sure to secure your camera with a strap first before doing so.
A wide-angle lens will allow you to snap a vast perspective of the sprawling city.
To keep a balance of architecture and landscape, I think it's best to incorporate the River Thames in the bottom of the shot.