Im sure there are other risks. With the bulk of the buyers in Knightsbridge now being foreigners, the safe haven for their money might not be so safe. Prices have already rocketed from 10 years ago, so further appreciation lies at the hands of the international rich. Prices in London as a whole are very expensive now compared to outside London (typically the ratio was 1.2 - 1.5 times in London to the rest of England, now the ratio is 2 times), so this might mean revert. The next government is looking likely to be Labour, and led by Ed Miliband, who politically is left of any Prime Minister in the Uk since 1979. He has said that he wants to bring in a mansion tax, which will bring much higher housing taxes and should bring about some sellers.
I agree there is potential there if the rich keep getting richer and prime central london is a place where the ultra rich want to live, but just wanted to point out that although it seems like prices in London can only go up, that has been said about many other bubbles in the past.