Canon lenses have an international warranty. They do not have to be purchased in the UK.
I have had to show a "proof of purchase" when taking kit in for warranty service/repair/adjustment, but I have no idea how much attention they pay to whether or not VAT has been paid at the point of sale. Since the warranty is international I don't really see that it's any of their business. If the lens had been bought abroad by a non UK resident there would be no need for VAT to have been paid. Surely all they need is a purchase date to ensure the product is within warranty.
I had no problem with a 100/2.8L IS Macro lens supplied by Kerso. Anything else I've taken in to date has been UK sourced, so not an issue, or out of warranty, so also not an issue.
p.s. even in the worst case scenario, where they might refuse warranty repair, not that they would have any right to do so, I could afford to pay for two fixed price repairs (£127.50 each time) to my 135/2 and still be no worse off financially than buying within the UK.