The UK Border Agency is responsible for securing the UK, controlling migration to the UK and enforcing immigration and customs law.
The Ministry of Justice is responsible for the justice system - sentencing, the courts, prisons and probation services.
Directgov has all the information you need on how to apply for a passport.
The Independent Police Complaints Commission is responsible for ensuring complaints about the police are dealt with effectively.
The Home Office is the lead government department for immigration and passports, drugs policy, counter-terrorism and police.