I believe that the prices will continually go up in London for rental property in the next 2-3 years due to the recent influx of land development going on there. This coupled with the fact that many Americans have taken a liking to the area and use it for a second home.
Rental costs in London and indeed in other major cities across the country and across the globe have been consistently rising over time and therefore the cost of rental properties in London will continue to rise over time. As long as people are prepared to pay the cost of living, costs will rise and as workers need to live somewhere, they will continue to pay.
You see, there's this thing called The Law of Supply and Demand. The theory behind it is simple, if the supply is greater than the demand, the cost decreases. However, if the demand is greater than the supply, then the price increases. The fact of the matter is that London is a major metropolitan area, and such is the case the world througout, these areas tend to have higher population densities than those more suburban or rural. And where there is a high population density, there is a high demand for housing. Unfortunately, housing is a finite resource and it simply cannot accomodate the needs of an ever growing population.
If the cost to rent property in London continue to go up, I believe it will not be sustainable. Many people cannot afford to live alone because rent is so high compared to earnings. As a young individual the opportunity to have their own rental it is very difficult considering the high costs and low income.