Nationals from the following countries need a visa before arriving to the UK: Afghanistan, Albania, Algeria, Angola, Armenia, Azerbaijan, Bahrain, Bangladesh, Belarus, Benin, Bhutan, Bolivia, Bosnia Herzegovina, Burkina Faso, Burma, Burundi, Cambodia, Cameroon, Cape Verde, Central African Republic, Chad, People's Republic of China (unless they hold a passport issued by Hong Kong Special Administrative Region or Macao Special Administrative Region), Colombia, Comoros, Congo, Cuba, Democratic Republic of the Congo, Djibouti, Dominican Republic, Ecuador, Egypt, Equatorial Guinea, Eritrea, Ethiopia, Fiji, Gabon, Gambia, Georgia, Ghana, Guinea, Guinea Bissau, Guyana, Haiti, India, Indonesia, Iran, Iraq, Ivory Coast, Jamaica, Jordan, Kazakhstan, Kenya, Korea (North), Kosovo, Kuwait, Kyrgyzstan, Laos, Lebanon, Lesotho, Liberia, Libya, Macau, Macedonia, Madagascar, Malawi, Mali, Mauritania, Moldova, Mongolia, Montenegro, Morocco, Mozambique, Nepal, Niger, Nigeria, Oman, Pakistan, Palestinian , Territories, Peru, Philippines, Qatar, Russia, Rwanda, Sao Tome e Principe, Saudi Arabia, Senegal, Serbia, Sierra Leone, Somalia, South Africa, Sri Lanka, Sudan, Suriname, Swaziland, Syria, Taiwan (unless they hold a Taiwan passport which , includes the number of the identification card issued by the competent authority in Taiwan), Tajikistan, Tanzania, Thailand, Togo, Tunisia, Turkey, Turkmenistan, Uganda, Ukraine, United Arab Emirates, Uzbekistan, Vatican City, Venezuela (unless they hold a Venezuelan passport which contains biometric information held in an electronic chip),Vietnam, Yemen, Zambia, Zimbabwe,
The territories formerly comprising the socialist Federal Republic of Yugoslavia.

Everyone else can receive a six month visa on arrival, expect to be questioned on entry if you are not an EU citizen.

United Kingdom - Visa information website

UK Missions

Foreign embassies in the UK