It won’t! The UK has 2 years after it has invoked Article 50 (the “Exit Clause”) to fully leave the EU. Until then, it is still a member just like it is today. Since fulfilment will be complete before two years is up, we don’t have to worry about this. Since the UK has yet to invoke Article 50 we will also have 2 years from the time they do to complete fulfillment prior to their membership status changes, giving us a substantial window to work with in the unlikely event of a delay. And, of course, even if it did matter, shipping from England to within the EU will still be cheaper and easier than shipments from the US would be.