You can continue to use Build International Listings on Amazon as normal after Brexit. The functionality of the tool will not change, but you might want to adjust your pricing rules to reflect changes in cost.
Remember, all list prices must include any customs duties. You can make edits to your international pricing via Inventory > Sell Globally > Build International Listings on your Sellers Central.
You can also adjust prices on a SKU-by-SKU basis by using flat files or going to Inventory > Manage Inventory > Edit listings.