Sorry, but no amount of selective colour is changing anything.
If an image is weak, then processing is NOT the answer.
Someone suggested the 12 o'clock light has not helped, but in reality, it's an overcast day so the time of day is irrelevant - it would have looked the same at 2pm, or 4pm (assuming it was still overcast). It's just dull. The bus dominates the frame, and making it back and white while leaving the rest colour doesn't help at all... then it's just as noticeable because it;'s black and white.
You can get it perfectly straight in Photoshop if you want, but then you'll lose a great deal from the edges of the frame, and as it was a tight composition to begin with, you've not a great deal of room to play with.
You'll struggle to get an original shot of the subject, simply because it's perhaps one of the most photographed things in the world.. no matter where you stand you'll find an almost identical image on Flickr I reckon, so let's leave originality out of this to make it simpler.
The bus is a distraction. I know it's an icon of London... a red bus... but it's one of those ugly new ones.. not a routemaster, so I would suggest it's just making things look messy, and not really helping. Unless you thought "old and new" was a concept you were going for here... but if so... it's weak, as the bus seems like an intrusion into the frame, not a considered inclusion.
The dull overcast day is making it look dull, and it's not a subject that suits such light. There's nothing you can do about that unless you go back and reshoot. You certainly can't do anything in processing to change the light.
Think about what makes the building interesting. Look at that detail in the architecture... you're losing it with the ultra low contrast lighting, but that white, flat sky makes it even worse because it makes the building even duller and drab by contrast..
I found this on Flickr... not a great photo, but look at the light and how it reveals the detail in the structure.
https://farm7.staticflickr.com/6121/5976396558_dc9fb4ac9a_o.jpg
Great images need great light, and your just doesn't have it. Simple as that.
I think there are better locations to shoot from too. It can be seen for quite a distance.... so you have South bank, the bridge itself, the water gardens.. loads of places.
I just think it's a snap shot, and as such, will never be much more, no matter what you do with it.
Think about angle... position.... Did you spend time walking around, looking at it through the viewfinder from different locations?
Processing is not the way to improve photography.
IMG_2654 by bobleeuk, on Flickr
london bus_NEW2 by bobleeuk, on Flickr
