Another factor is that lack of decent jobs in the regions and rural areas means young people can't live at home anymore. Many have to move and get their own flats because the only available jobs are in large towns and cities far beyond commutable distance from their family homes,
It's why places like Cornwall and the Lake District struggle. The only jobs are retail and hospitality, and people wanting a better career have to leave their home towns and move to larger towns and cities. Places like London, Leeds, Manchester, etc are sucking in younger people but aren't providing adequate housing for them. A vacuum is left in the regions filled by people wanting to invest in holiday homes. The London centric nature of the country over the past few decades, centralisation of jobs into London and a handful of other large cities, wholesale closure of regional offices, local branches, etc., has greatly contributed to the housing crisis in different ways.
If my son could have got a job within commuting distance of our home, he'd be living at home as he's single. As it is, there are simply no employers in his financial services profession within commutable distance of our home, so he had to move 3 hours away, pay 3/4 of his wage on rent, utilities, travel, etc., just to work. A generation ago, our area had two large financial services employers - both closed down with staff made redundant or relocated to their London office! My son basically "crashes" in his flat during workdays and comes home most weekends and holidays etc. - he has no social life there, it's here. What a colossal waste of money (and travel time) just so that he can work in his profession - taking up a home that a more local person could benefit from. Lots of his colleagues were living in hostels during their first few months after starting work with him as they literally couldn't find anywhere to live, no flats, no house shares, etc - demand so high that estate agents were only giving an hour of viewings for each property/share and landlords had the pick of applicants due to the insane high demand.
So much has gone wrong with this country over the past few decades, from London Centricity, lack of housing, immigration, holiday lets taking residential properties out of the market, etc. Massive changes are needed in so many areas.