It's very unclear from your question what you are doing, but let me lay out the salient points. Assuming you are in the US legitimately:
As a UK citizen you are permitted to enter Canada without a visa for short visits. If you show up at the border you should be let in.
Likewise the US will permit you to return after your trip to Canada without any other kind of permit.
However, if you are nearing the end of your permitted period of time in the US, a day trip to Canada will NOT reset the clock. Your permitted length of stay will be as if you had not left the US.