Here are some helpful links to determine the necessary documents you will require in different situations:
Visiting the UK:
http://www.ukba.homeoffice.gov.uk/visitingtheuk/
Studying in the UK:
http://www.ukba.homeoffice.gov.uk/studyingintheuk/
Working in the UK:
http://www.ukba.homeoffice.gov.uk/workingintheuk/
Settling in the UK:
http://www.ukba.homeoffice.gov.uk/settlement/
While you are in the UK:
http://www.ukba.homeoffice.gov.uk/while-in-uk/
To speak with someone regarding your query:
http://www.ukba.homeoffice.gov.uk/contact/contactspage/contactcentres/
