The aim of a customs union is to facilitate free trade between Member States. The union reduces the administrative and financial burden of trade barriers and promotes economic cooperation between nations. If we left both the internal market and the customs union, we could negotiate a free trade agreement with the EU. A free trade area is an area in which there are no tariffs or taxes or quotas for goods and/or services from a country entering another country. However, multilateral and bilateral approaches – removing trade barriers in coordination with other countries – have two advantages over unilateral approaches. First, the economic benefits of international trade will be strengthened and strengthened if many countries or regions agree to remove trade barriers. By expanding markets, concerted trade liberalization enhances competition and specialization between countries, increasing efficiency and consumer incomes. In the 19th century, Britain also benefited from the unilateral reduction of its tariffs, as its success in free trade led other countries to lower their barriers. The effect of the creation of trade is generally seen as a positive effect. This is because the domestic production costs of Country A exceed the cost of production of country A imports from Country B. The customs union allowed Country A to abandon domestic production of certain goods and turn it into Country B to produce these goods.
From a global perspective, this type of production conversion improves the efficiency of resource allocation.  In other words, in Country B, not all products imported from Country A are duty-free, but only those that comply with the rules of origin. There are many variations in the rules of origin. In country B, for example, only goods with more than 40% of the value added in country A (or country A-country B) will be subject to the free trade agreement. When the effects of trade liberalization are discussed through free trade agreements, declarations often focus primarily on the trade volumes of both countries. However, this is a bad approach. In fact, the only ones to benefit from a free trade agreement are those that were initially subject to tariffs and are compliant with the rules of origin and are therefore exempt from tariffs as a result of the free trade agreement (there are exceptional cases where tariffs are still charged, but at a lower rate). Once goods are subject to tariffs in one country, they can be shipped to other EU countries without further customs duties. In addition to the advantages, customs unions also have some drawbacks: although virtually all economists believe that free trade is desirable, they differ on how best to move from tariffs and quotas to free trade.
The three fundamental approaches to trade reform are one-sided, multilateral and bilateral. Similarly, services are a large part of the UK economy, 78%, and are not fully covered by a customs union, because they tend to carry “non-tariff” barriers. (i) tariffs and other restrictive trade rules (with the exception of tariffs permitted by Articles XI, XII, XIII, XIII, XIV, XV and XX) are abolished primarily for all trade between the contracting parties or, at the very least, with regard to all trade in products originating in those territories, and the advantage of these bilateral or regional agreements is to promote greater trade between the contracting parties.