I think that besides taxes and fees, the main issue - and bottleneck - will be customs. In EU you have free movement of goods and people, so if I buy from Amaz

I think that besides taxes and fees, the main issue - and bottleneck - will be customs. In EU you have free movement of goods and people, so if I buy from Amazon DE, FR, IT, the item will be shipped and moved around EU territory and easily get delivered to my door. If I buy from Amazon.com I'll get an email from the customs where they ask me for proof of purchase where the proper taxation/custom fees were paid. This is what will happen in with products coming from the UK. This means the package is being processed by a foreign entity with no time window to be released (because part of it will depend on you as well). Amazon simply cannot keep UK in their european fulfillment network warehouses because it will have items go through customs. It would be a bottleneck in a seamless movement of products. It's not about geography, it's about crossing borders.