Here is the statement from Amazon on UK and EU marketplaces;
"On January 31, 2020, the United Kingdom left the EU and entered a transition period in which existing arrangements will remain in effect until December 31, 2020. The United Kingdom will formally leave the EU's internal market and customs union on 1 January 2021. While negotiations between the United Kingdom and the EU are underway (including setting tariffs, if any), from January 1, 2021, there will be a customs border between the United Kingdom and the EU that will affect businesses crossing this border to work. Amazon is committed to supporting the sale of your business in the UK and across Europe. Although your Amazon business will continue to operate as usual until January 1, 2021, this page explains what actions you can now take to prepare your company for the new customs borders. This page is regularly updated."
Do I have to create a separate SKU from 01 January 2021; one for my UK listing and one for my EU?
No. You can continue to use the same SKU for your UK and EU listings. However, if you want to ship to both sides of the border, you must enable multi-country inventory. It allows you to bring your FBA inventory to Amazon distribution centers in multiple countries of your choice across Europe. Once enabled, you can choose which country to ship to. You can use the same SKU for your UK and EU shipments and we treat them as two separate inventory pools.