Beautiful picture. There's still a little more detail on the subject than I would like to see in a silhouette. The colors are gorgeous and the sky is just superb!
I'm glad you didn't go for the pitch black silhouette - the slight detail works very well. The sunset is very nice too, with plenty of cloud to add interest and a pleasant change from the usual garish oranges & reds. The yellow seems a little odd to these eyes, but it is London so I guess anything is possible!
What really makes this shot though is the boat. Perfectly placed in the "less interesting" side of the water, it give the whole shot that little extra. Superb.