I second Greenwich , (but I am biased , I live here) but for a posh lunch with a view either the Shard (very expensive though) and the view can be obscured by the clouds and haze , but much better in my opinion is the Oxo Tower on the South Bank.
There are 2 restaurants side by side on the top floor , one is the Brasserie which is better value , the other in the full blown Harvey Nichols job. If expense is no object then go for the latter , but the view is the same from both and the food is excellent in the former. I use it for special occasions.
