Britain's Foreign and Commonwealth Office (FCO) offers country-specific travel advice notices for British citizens; these notices are regularly updated.
The FCO site also explores in depth the risk of terrorism when traveling abroad.
The U.S. State Department offers travel advice for U.S. citizens on its web site, including information on travel to the United Kingdom.
In 2004, the US Department of Homeland Security developed a scheme at various US air and sea ports to register the departure of non-immigrant visa holders, this was extended to cover travellers on the visa waiver program from September 30, 2004. The FCO's travel advice for the US has more information.
We hope this answered your inquiry. If not, contact us using the email form and we will reply to you within twenty days.
Note: The British Government is not responsible for the content or reliability of external websites or the websites of other governments and does not necessarily endorse the views expressed within them.