What's the situation of social work in UK? In JPN, it has been not matured yet.?

Answer 1

Every area has social services administered by the local authority. The social services departments are usually split into two divisions: (1) adults, (2) children & families. Adult social services are split into four areas: (a) mental health, (b) learning difficulties, (c) physical & sensory disabilities, (d) old people.
Services for adults include domiciliary care, meals-on-wheels, day centres, residential care, advocacy, respite care etc. For children & families they include: adoption, fostering, children at risk monitoring, children's homes, respite care etc.

Here are two very different local authority websites for you to have a look around.
London Borough of Camden http://www.camden.gov.uk/ccm/navigation/social-care-and-health/about-social-care-in-camden/
Buckinghamshire County Council http://www.buckscc.gov.uk/bcc/section/index.jsp?contentid=-1498585593