Hello, I´m from Spain and live in USA. I have crossed the border in both directions several times. You can cross the border without any problems with your passport. If you are a resident in the USA, bring your residency card. If you are a tourist, there are 2 case scenarios:
-if you plan on leaving for the UK in a plane from the USA (wich means you have to re-enter back into USA to catch your flight, boat...) bring a copy of your plane ticket to show to the canadian officer, so they do not take your I-94 form from your passport and then, you can re-enter back into the USA without questions, new paperwork, etc...
- if you do not plan to re-enter to the USA, you´ll have to give them your I-94 form (they´ll give it to the USA officers, they´ll stamp your passport, and that´s all.)
This works for some european country's citizens and some others (Australia, New Zealand...) exempt from entry visa in the USA/Canada and with a valid I-94 (the one you fill in while in the plane and approved at the airport at your arrival into the USA.) For other countries not part of the visa exention program, assuming you have the propper documments, ask the canadian officer to let you chat with the US officers before stamping your passport and officially entering Canada, just to make sure the US officers will let you back in.
I ussually find canadian officers a little more friendly that the ones in the USA, but lately, them all try to be very helpfull and nice to tourists. Again, copies of tickets showing plans on where/when leaving which country, makes things very smooth, as immigration officers rather like to see documents that having to listen to you for a few minutes.
Hope it helps.