There are fewer than a quarter of a million long term empty homes in England, sp. Stats here from what should be an unbiased source:
http://www.emptyhomes.com/statistics-2/empty-homes-statistice-201112/
Even assuming that all of these are habitable and could be used immediately (and that's extremely unlikely) this represents about 18 months supply to cater for immigration. This supply once used will not be replenished.
To blame people who buy property and leave it unoccupied is unfair. People are entitled to do as they wish with their purchases and I have no wish to live in a country where the government tells me what I can and cannot do with my acquisitions. Whatever the reason there is a housing shortage in the UK and in England particularly. How any country can bleat on about the lack of housing (affordable or otherwise) whilst being stupid enough to allow upwards of half a million people a year to settle on its shores defies logic. The answer to this shortage is not to build more and more homes, many of which will have to be funded by somebody other than the occupants. The answer is to restrict the number of people arriving here.