If large businesses owned a huge number of rental properties, it would be politically easier to regulate them -- limiting rent increases, requiring insulation etc.
Conversely though, it would also be easier for them to lobby the government to deregulate the rental market. A company with £50bn of housing stock on its books would be very powerful.
Even ignoring that though, and assuming that companies could be regulated well, you'd still be looking at a situation where all young people would be transferring most of their wealth to the owners of these companies forever. There would be no way for people to use property investment to fund their retirement, people would never feel secure enough to have kids, and ultimately whether or not people would be able to live in an area would be at the whims of whether a business will rent them a home. It's a massively dangerous situation for a society.
People use the equity in their homes to do things like funding startups. Loads of successful business started out with founders mortgaging their properties. This is would bring about the end of that being an option.
