Ribble cycles: Cheapest one, no international tracking number but it's free and fast delivery, but some items are not available internationally (liquids, lubes, bike trainers, big items etc.), they are really good overall. This is my favorite place so far, they usually give the best price.
Chain reaction cycles: Not the cheapest, but they provide tracking number. They sell big items internationally with free shipping. Their customer service is good as well.
Jenson usa: Expensive, but they pay duties on behalf of you, so you don't have to deal with customs (important fact if you live in Canada).
US stores are usually expensive, I don't know why. GB sellers convert GBP to CAD (or any other currencies), so it is easier for international buyers to see what they are going to pay.