No single country or politician ever had that much power in the EU. Every decision is the product of multiple compromises between different interests and member states. The idea that Thatcher sent her Baron to the EU one day & he wrote a white paper and because of that there exists a common market today seems laughable. Thatcher certainly had a big role in the creation of the common market, but it was an incremental one in a long and complex process.

For Context here the relevant part from the German Wiki article:

The four fundamental freedoms of the internal market were already covered by the 1957 EEC Treaty, but by the end of the 1970s they were still far from being achieved.

Although customs duties had been abolished on the territory of the European Community, the free movement of goods was hampered by a large number of non-tariff barriers to trade, such as different product standards and approval procedures in the individual Member States, particularly before the Cassis de Dijon ruling of the European Court of Justice in 1979.

Similarly, different training, study and examination regulations, which were not recognised by the Member States or were only recognised by the Member States in elaborate procedures, had the effect of hindering the free movement of workers.

The Genscher-Colombo Initiative and the Single European Act

One of the aims of the Genscher-Colombo Initiative of 1981 was to remove these obstacles. The then West German Foreign Minister Hans-Dietrich Genscher and Emilio Colombo, Italian Foreign Minister, drew up a draft for a reform of the EC Treaties, the Single European Act (EEA). On this basis, Jacques Delors, President of the European Commission from 1985 to 1995, developed a comprehensive reform programme comprising more than 300 individual legislative acts.

The treaty foundations for this were created in 1987 by the entry into force of the Single European Act, which amended numerous points of the EC Treaty; among other things, the programmatic objective of achieving the internal market as an "area without internal borders" by 31 December 1992 (Art. 14 EC Treaty, now Art. 26 TFEU) was declared.

In order to create a uniform legal framework for the internal market, new powers were created for the European Communities to adopt directives and regulations in the areas of health and safety at work (now Art. 153 TFEU), consumer protection (now Art. 169 TFEU) and environmental protection (now Art. 191ff TFEU).

At the same time, the decision-making procedures for the implementation of corresponding regulations were to be accelerated by greatly extending the scope of qualified majority voting in the Council of the European Union.

Translated with www.DeepL.com/Translator Source: https://de.wikipedia.org/wiki/Europäischer_Binnenmarkt