Since more people LEAVE the UK than enter (even with EU expansion) how long will it take house prices to FALL?

Supposedly the incoming migrants are pushing up prices, BUT this assumes net population is rising, IT IS NOT ! since more people are leaving for a new life in Spain, Australia etc. maybe they are renting their house but soon enough with high mortgages and the need to buy elsewhere they will have to sell and when many people sell, prices will fall, so the question is HOW LONG before this happens ?