You should have a valid ESTA. A valid ESTA enables you multiple entry into the US from the time it is approved to a period of 2 yrs or until your passport expires.
At some Canadian airports you pre clear US Immigration/Customs in Canada so you arrive in the US as a domestic passenger.
Good idea as well is that when you return from Canada to US that you also carry with you proof of your return ticket US-UK
As long as your ESTA is in order (which it would be if you enter the US initially) you shouldn't have a problem. If you're flying back from a preclearance airport (eg Toronto) make sure you get to the airport well ahead of your departure (I'd say 2.5 hours). Preclearance at Toronto can be very busy and take a long time.
I'll make sure I carry all my paperwork and evidence of onward travel and make sure I get to Toronto airport in plenty of time.
Thanks for the advice :)