The only things I prefer not to buy grey are products with a relatively high failure rate like third party lenses (Sigma UK's three year warranty is worth having, though still inferior to the warranty period in other countries) and any flashgun.
Also if Canon overseas sales are priced in USD they may require further cushion if GBP falls vs USD - significant falls could happen in the current financial climate.
Anyway price differential still looks excessive!
Try say New York shop to see whether they will sell you one without local sales tax (because its an export).
Airmail would cost very little for such a tiny lens. Then add import duty and customs charges on the UK side - then VAT on everything so far. Could save a bit! Only worth it if lens has an international warranty.
My guess is there is another factor: the high yen.
The yen is so high that Japanese mfrs are not making much money in the States. They need to keep prices near where they used to be, but margins are cut. That leaves them trying to make money anywhere else they can.
Anyway, I live in the UK and you can't just do the exchange rate to figure out how much things cost. Things are more expensive here - that's how it is.