The promotion period is 15 March 2005 to 31 December 2005. The products...must have been bought in this period to be eligible for the Cash Back.
The promotion runs in the following 14 countries: Austria, Belgium, Finland, France, Germany, Greece, Ireland, Luxembourg, The Netherlands, Portugal, Spain, Sweden, Switzerland, or the United Kingdom. For the Cash Back to apply, the products on the above list must be purchased by an end-user in either of those countries.
Appears to be no probs with lenses bought in the Channel Islands if you're an end-user based in the UK. The wording doesn't suggest that the product itself must originate from those listed countries.
http://www.canonextrapromotion.com/terms.aspx