Personally, I recommend York, Lancaster and UEA, as they consistently rank very highly over the years.
Recent rankings:
http://www.guardian.co.uk/education/table/2009/may/12/university-guide-social-workMore information:
http://www.guardian.co.uk/education/universityguidehttp://www.guardian.co.uk/education/table/2010/feb/15/social-work-postgraduate-masters-table1http://www.guardian.co.uk/education/2010/feb/15/postgraduate-tables-2010-methodologyhttp://www.guardian.co.uk/education/socialworkPlacements and lack of them are also a huge issue.
I think you might notice that in some of the tables provided above, the percentage of international students at the universities in this subject area is very low, from 0% to 20% maximum. This is because the financial support and funding arrangements for students is geared towards British and EU citizens and residents and those with ILR.
Having British social work degree will not guarantee you work in the UK necessarily on graduation, though at the moment, there is a post study work visa, which is good for 2 years and then you'd have to switch into another category, keeping in mind the immigration system is a rapidly changing beast. You'd also need to be sure and clear that the degree you did would be recognised in the US, as you wouldn't want to invest all this time and money, only to not be able to practice in the US, if for whatever reason, you returned to the US.
I think you'd be better off doing your MSW at an American university and then registering with the Social Care Council in the area of the UK you'd like to work and then apply for jobs to get a (current) Tier 2 work permit in the shortage area only of child protection, providing that immigration track remains.
Another option for a change of scenery and to get around the degree recognition issue in the US is to do your MSW at a Canadian university and then registering with the Social Care Council in the area of the UK you'd like to work and then apply for jobs to get a (current) Tier 2 work permit in the shortage area only of child protection, providing that immigration track remains. Providing that all works well, you could have the experience of two other countries. With a North American MSW, you also have the NAFTA option to work temporarily in Canada and if you do your degree in Canada, there is also a similar post study option.
See the information in the link in my first post. It's also worth posting your plans / ideas in the Calling all Social Workers thread to get input from American MSW qualified social workers who are in various stages of the process you describe.
HTH.
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ETA: have you considered BUNAC:
http://www.bunac.org/usa/interninbritain/ to get a flavour of living in the UK?
Or are you eligible for citizenship of an EU country?
http://europa.eu/ to live / work / study in the UK:
http://www.ukba.homeoffice.gov.uk/eucitizens/