You'll get directed to the UK site based on you IP address normally. If you want to go to the US site then go to the "Select an International version" drop down menu in the bottom left corner and select "USA: English".
You can tell whether you're on the UK or US sites in two ways easily - the currency they use for the offers they advertise and, in the box where you search for holidays, how do you input the date? If it's Day/Month/Year you're on the UK site; Month/Day/Year and you're on the US site.