No go areas:
The British Foreign Office and Commonwealth Office website, www.fco.gov.uk, recommends British citizens avoid: Burundi and Liberia.
In addition, parts of (detailed on the website): Afghanistan, Albania, Algeria, Azerbaijan, Chad, Colombia, Congo, Eritrea, Ethiopia, India, Indonesia, Israel and the occupied territories, Ivory Coast, Kyrgyzstan, Nigeria, Philippines, Russian Federation, Sierra Leone, Somalia, Sri Lanka, Sudan and Uganda.
The US Department of State's Bureau of Consular Affairs' website, www.travel.state.gov, recommends US citizens avoid:
Saudi Arabia, Lebanon, Libya, Turkey, Sudan, Bosnia, Iraq, Somalia, Nepal, Israel, the West Bank and Gaza, Angola, Liberia, Kenya, Algeria, Indonesia, Yemen, Ivory Coast, Burundi, Afghanistan, Nigeria, Colombia, Iran, Pakistan, Central African Republic, Congo, Zimbabwe and Tajikistan.