Comment by sshine

There are no documented cases of the UK intercepting and arresting a US citizen traveling through Europe for an act that is not a crime in the United States.

Extradition treaties, such as the UK–US Extradition Treaty of 2003, allow for extradition requests between the two countries.

They generally require that the alleged offense be a crime in both jurisdictions ("dual criminality") which ensures that individuals are not extradited for actions that are not considered crimes in their home country.