From Wiki

The British government has a useful website called Moving back to the UK : checklist.

Department for Work and Pensions

Foreign and Commonwealth Office

VISAS, IMMIGRATION & NATIONALITY
http://www.ukvisas.gov.uk
http://www.workingintheuk.gov.uk
Home Office
Passports
http://www.oisc.gov.uk
http://www.jcwi.org.uk
http://www.ias-uk.org

OTHER GOVERNMENT SITES
http://www.ons.gov.uk (National Statistics)
http://www.royal.gov.uk
http://www.bankofengland.co.uk (UK Central Bank)
http://www.hmso.gov.uk (Legislation Information)
http://www.parliament.uk
http://www.caa.gov.uk (Civil Aviation Authority)
http://www.inlandrevenue.gov.uk (Taxation)
http://www.hmce.gov.uk (Customs & Excise)
http://www.ta.mod.uk (Territorial Army - UK Army Reserve)

The GRO's ( for people who need to get birth certficates etc):

England and Wales
N. Ireland
Ireland
Scotland