What is the point of buying the house in UK if I could be free social housing?

Social housing you would have to live in a council area for three years before being eligible to get on the housing list, it could be ten years ( or never) when you are offered a place and only if you fit the criteria.

Young children the council are responsible for housing ( not the parents) so they "may" offer accommodation and that might be B&B or a hostel and people can live in one room for years before they are offered a home, if the offer is "unsuitable" for you and you turn it down then they have no obligation to offer you anything else.......... and not sure why you think you can live for free, people pay rent ,yes there is HB for those who are low income and thankfully new laws are being put in place to stop greedy people who can afford to support themselves to pay market rent from social housing.

If you buy or rent, you get to choose where you live (according to your finance system)

With council housing, if you ever qualify, you don't, nor are you guaranteed to be able to live there for ever. And it's not free; its subsidised.

You can apply for council housing; however if you're not a priority, you may never actually get to the top of the list. Homeless, vulnerable and families will always be ahead of you.

You seem a little out of touch with reality - benefits are being reduced so the idea of having children to get a terrible house in a bad area is probably a non-starter. £300,000 will buy an excellent house - if you wwant better work to improve yourself and earn it