As far as the first question goes we only need to consider the signature and effective brand of the architect. It's as simple as whether you have ever heard of them or not. Richard Rogers, Norman Foster, and the late Zaha Hadid, with perhaps Sir Terry Farrell and James Stirling tossed in for good measure, make up a cross section of British architectural brands. What's interesting is how few buildings all of them have in this country… but can you blame a brand for wanting to expand?
Contrast this with the endless new flats being pumped up everywhere in London's residential gold rush. Any brands here? No, and that's not to say that they are all bland functionality, quite the contrary, but without a brand involved they're just so many new, flashy places to live. No tourists will come specially to see them, will they? The real sculptures are the ones that show up in the skyline silhouettes like Foster's Gherkin.
Which brings us to the next question, about when a building should be considered a sculpture first and an edifice second. Traditionally there have always been follies, such as the Dunmore Pineapple, where the purpose, if any, of the building was completely superseded by its sculptural qualities. Perhaps from a modernist, functionalist point of view we might consider some white elephants as sculptures, such as one finds in Olympic villages of yore, where they have lost their prime function.
We can also consider pioneering work that often cost too much and had too many technical problems from boldly going where none had gone before. For example several of James Stirling's buildings, such as the revolutionary Neue Staatsgalerie in Stuttgart, suffered from technical problems from using new and untried materials. Seals were a problem with his use of large-scale glass, causing many leaks and temperature problems. They were nonetheless trophy buildings still famous decades later.
Spanish architect Santiago Calatrava gave his native Valencia a whole park of magnificently sculptured trophy buildings that dazzle the tourists. The locals, alas, complain endlessly about their cost, which they are still paying for, and the fact that some of them turned out to be much less useful than planned. Some of them are effectively white elephants. Probably Frank Gehry's Bilbao Guggenheim Museum is the ultimate brand example, as the building itself was so popular that people in many other countries wanted exact replicas of it, presumably to replicate the mystique and success that went with it.