Safe to assume you will get hit with duties if the item is over $50, I've gotten hit with duties from End. and some other site (for just a t-shirt). Usually duties are 13% tax + whatever brokerage/duties the carrier feels like charging. Brokerage/duties vary between the various carriers, if it comes by way of Canada Post (whatever the UK post office is called, it will be handled by Canada Post once it lands in Canada) that will be as fair as it gets. Anything else, be prepared to be hit with $20+ brokerage/duties and 13% tax. You can do the brokerage/importing by yourself, you will have to go to a Canada Customs Office and clear the items, did this a couple of times and they only charged me the 13% tax on the item's value, they can charge duties on top too, but usually they don't care.
Because of this, I find it more convenient to just ship packages from Europe/Asia to my US mailbox, I've never been hit with duties when shipping to the US.