Digital cameras are zero rated for duty, but you still have to pay VAT.
So:
1) Take the total cost of camera + shipping and work out how much that will cost you in pounds. I use this link: Currency converter
but most credit card companies, or Paypal, charge you at 3 or 4% worse than the standard exchange rate.
2) Add 17.5% to the total
3) The courier (including Royal Mail/Parcel Force) charge you a fee for dealing with customs. The last package I got, worth around £180, cost me £13.50 to collect but they may charge more for higher value items.
Also bear in mind that you won't have a UK warranty - Canon warranties on digital cameras only apply to the country of origin, so you'd have to send it back to the USA for any repairs or recalls. That seems a little risky on a new model of camera, but only you can decide if it's worth it.