What is the US doing in UK?

I am not going to get into the debate over the nine-strong Muslim family stopped from boarding a flight from Gatwick to Los Angeles to see members of their family and Disneyland. What I would like to address is that there is an underlying story to this which begs questions which need to be answered. Primarily, that is what are members of the US Homeland Security (FBI) doing on active service in the United Kingdom? This story has blown their cover. For example, when members of EU police forces come to Majorca every summer, they come in an advisory role because, by law, they are allowed to operate as police as they would in their country of origin. Only in special and serious crime cases which cross boarders do foreign police forces co-operate and act together. OK, the call to stop the family came from Washington, but could not the security services in the States have let the UK border control agency know and let their people  deal with it?