Only way house prices will come down
a) ukip get in and leave the eu and put a stop on mass immigration, putting a hold on increases in demand.
b) have another economic recession which will lead to immigrants to go back to their home country, reducing demand on housing.
c) massive increase in supply of housing (unlikely due to the state not allowing it)
d) new laws that prevent foreign ownership on housing like in arab states, reducing demand and increasing supply.
e) have another economic recession which leads to interest rates to go up and push a lot of people in to unaffordable mortgages, increasing supply as they sell and reducing prices as supply outstrips demand.
immigration has little if anything to do with house prices across the country.
if anything immigration has a negative affect on house pricing by providing cheap labour resources for new builds.
But you are right, if UKIP got in to power (and pigs fly over a frozen hell) then house prices will collapse following a high recession. Of course with the economy shattered affordability will be far worse.
As for foreign owners - they may locally drive up prices in some parts of London, but it is not a nationwide issue at all. Plus the UK greatly benefits from the foreign investment and taxation of property.