Strangely I had bigger problems travelling from the US into Canada on a Greyhound bus, we had to get off the bus with our luggage and all go through passport control on foot.
Echo the others as a UK citizen all you need is the ESTA do it online and get it approved in minutes (can be a couple of hours) do not use any other site than the US government as they charge a "service charge" which is not necessary, they were valid for 2 years.