They include the scarcity of suitable land, the British obsession with house ownership as an appreciating investment, the policies of developers and the construction industry, the decline (through the policies of successive government since the 1970s) of available social housing, the increasing number of single-person households due to the fragmentation of the traditional family, second home ownership in rural areas, the planning regulations, etc etc etc. Among all these immigration (or more accurately the rising population) is certainly a factor, but actually a relatively minor one.
What is “social housing”?