Err... Thanks? But im just gonna buy it from Apple Store in the UK to save all the hassle.
Probably the smartest thing to do!
If you bought it from Canada,
This is basically what your totals (and hassle scenario) would look like:
iPhone 4 16gb, CAD $659
Ontario Sales Tax of 13% inclusive: CAD 744.67
Thus, for CAD 744.67, your friend will have the iPhone 4 in his hands to bring back to the UK for you.
Since, he is leaving the country and does not intend to bring the item back, he can apply for a refund of the GST/HST upon departure:
http://www.cra-arc.gc.ca/tx/bsnss/tpcs/gst-tps/rbts/vstrs/frms-eng.html
After completing the paperwork, he will get a refund of CAD 85.67
Now your friend boards the plane to LHR, and gets in to the UK. Lawfully, he should declare that he is bringing in a good of value greater than the free exemption, which he plans on leaving back in the UK. The free allowance is around 300 pounds (484.331 CAD).
He will have to pay VAT on the difference between the value of the good and free allowance:
659 CAD = 408.192 GBP
408.192 GBP - 300 GBP (free allowance) = 108.192 GBP
108.192 GBP * 17.5% VAT = 18.9336 GBP
So net cost of the phone (assuming your Canadian GST/HST refund application is accepted) would be 408.192 GBP + 18.9336 GB = 428.1256 GBP ~ 428.13 GBP
Cost of iPhone 4 16GB at the UK Apple Store 499 GBP incl. VAT
Net savings = 499.00 GBP - 428.13 GBP = 70.87 GBP
The value of the time, hassle, and inconvenience that could potentially be caused by undertaking this process = priceless
Hence the earlier advice from several, just get it from the UK.