Just back from our trip and i wanted to update everyone as there have been many questions and loads of confusion about crossing into the US from Canada with a UK passport.
What we found out was as we had originally come from the US as part of our holiday and had just gone to Niagara for a night or 2, when we crossed back into the US there was no problem, no fee and a simple welcome back to the US. We noticed that other people who were entering into the US for the first time did have to fill in some forms and answer various questions, but the US border control were very polite and did their best to hurry the process up. We crossed the border on foot.
Hope this helps someone