Most visits to the UK are trouble-free but you should be aware of the global risk of indiscriminate international terrorist attacks, which could be against civilian targets, including places frequented by foreigners.
This advice is correct at time of publishing. As the situation can change rapidly, visitors are advised to contact the following organisations for the latest travel advice:

US Department of State
Website: http://travel.state.gov/travel

The Ministry of Foreign Affairs in your home country.