Real Estate prices go down really hard, when the economy is in a downward spiral. Look at the situation in 2008-09. During that time, the prices for houses, apartments and residential plots went down by an average of 30-40% in the United States. In places such as Dubai and Sharjah, it went down by as much as 60%. Stay away from real estate in the emerging markets, in case of an economic slowdown.
In emerging markets, underwriting standards of banks are much more stricter than those of developed economies. Moreover, you can count on the government to intervene in the case there is a threat of market collapse. I would say the London real estate market is overheated now.