Hi!

First a short background, i won't go into much of the history here, just a few important bits. I talk a bit more about the backstory to the EU here

Background and general information

The EEC (European Economic Community), later EC (European Community) was established with the Treaty of Rome in 1957. In 1958 Charles De Gaulle became prime minister of France, and was generally opposed to European integration, for example illustrated by the empty chair crisis. De Gaulle also vetoed Britain joining the EU in both 1963 and 1967, and it finally did join in 1973.

It's important to know that the early EU (and the contemporary one) was always both a political and an economic project, and countries had varying preferences and motivations to participate in both. Some countries that mainly wanted economic cooperation founded the EFTA (European Free Trade Association) in 1960 as a parallell organisation to the EU, among those was Britain.

In addition to the EU, other organisations were established among the same time to foster European cooperation. An example is the Council of Europe, of which the UK has always been a part. In the contemporary post-war this howerver did not achieve as much as some had hoped, among other things because of opposition to anything that went beyond voluntary and limited cooperation, the UK foreign secretary Ernest Bevin for example commented, in relation to suggestions of giving the Council actual powers:

Once you open that Pandora's box, you will find it full of trojan horses

Britain initially participated in the negotiations leading up to the Treaty of Rome (what became known as the Spaak report), but withdrew from the negotiations in 1955 when it became apparent to them that the other countries did not accept it's hope of limiting the treaty to a loose free trade area, and this is what later sparked their participation in EFTA.

So from Britains actions we can see that they did want economic cooperation but generally rejected political cooperation or any integration. I am going to look at why they just wanted economic cooperation and opposed the early EEC and why they changed their opinion and applied for EEC membership already in 1961, rather than just stay in EFTA which they had helped create three years prior?

Britains worldview and culture, changing opinions on EEC membership

Britains position in the 1950s, initial opposition

Firstly a point on the british interests and worldview. Britain in the postwar saw itself operating within three different "spheres" of important relations:

(1) The Empire and the Commonwealth;
(2) The Atlantic Alliance and the "Special relationship" with the US and;
(3) their relationship with western europe.

Their importance to Britain was in the order i gave them here, western europe was less important than it's other relations. This is probably the central point of it's initial opposition. European integration was not it's foreign policy goal or focus.

Secondly a point on british culture. The british tradition of parliamentary sovereignty and to the extension of that, it's self-image as both an empire, and of not having been under rule or influence by foreign powers for centuries, led to a general view where concessions of sovereignty was neither good or necessary [1]. This can also be seen in line with, simplified, 250 years of British foreign policy doctrine: to prevent continental political unity. The British was also one of the few countries that had managed to safeguard their own democracy and independence, not seeing the same need as others to unite to defend it in the future.

Thirdly, all the initial political cooperation was in areas that didn't benefit Britain. The Coal and Steel community had no interest for it, as it's coal and steel far exceeded the others and it had a better defensive position anyway. The European Defence Community would have limited it's manoeuvrability when it's forces were already stretched working with the empire and NATO (who, by the above, were more important). Only the EEC (EC) was attractive for the economic cooperation, which means they only became interested in the European political project once it had more economic relevance for them.

Britains position in the 1960s, less self-confidence and changing opinions

It was quite quickly evident for Britain that it was no longer a global superpower, though this arguably didn't become clear before the late 50s or early 60s. The Suez Crisis of 1956 is typically taken as a changing point. More colonies also became, or were seeking, independence. This weakened the importance of (1) the empire, even if they still stayed in the commonwealth this could not provide the same political importance. Trade within the commonwealth was declining as well.

The special relationship had also weakened. The Suez Crisis was part of that, but also general disagreements, like the US sabotaging UK attempts at detente with the USSR in the 1960s Paris summit. The height of these issues was perhaps the public disagreements during the Skybolt crisis, where the US cancelled some nuclear cooperation and quite publically challenged the UKs attempt to try and play an independent power status internationally. It's worth mentioning that the US favoured a strong EC and pushed for UK membership. In summary, events lowered the importance of (2), the special relationship.

Furthermore the EC was actually doing quite well and the memberstates were outperforming the UK in many regards [2], like GNP, trade and investments. A part of this was also an increasing UK trade pattern with Europe. And while they had EFTA, it was quite clear that the EC was the driving force of economic integration in Europe [3]. To a degree, the EC was also a way of combating a worsening special relationship and it's relative decline in the world. This increased the importance of (3), their relationship with Europe

So in total, the late 50s and early 60s saw Britains international position and their relationship with the US become less important while they became economically more integrated with Europe, and the EC seemed like a success. These facts, quite simplified, held true until they could actually join in 1973. It's quite evident by the 1975 referendum that the people in general had also accepted positive attitudes towards the EU, with 67% voting in favour of remaining in the EU.

In total and in conclusion the primary motive seems to be economics, with European trade becoming increasingly more important to the UK, with the EEC/EC being an increasingly large part of that. There is something to be said for a political incentive in having European friends and not just the unreliable commonwealth and the dominating US, but the UK consistently opposed European integration, both before and after joining, so it's clear that European cooperation on a supranational level was not a motivation, though it might have been begrudgingly accepted in order to gain European friends and allies.

These points, and the change of opinion, can be illustrated well by an excerpt from a speech of the UK prime minister, Macmillian, in 1962.

We in Britain are Europeans. That has always been true, but it has now become a reality which we cannot ignore. In the past, as a great maritime Empire, we might give way to insular feelings of superiority over foreign breeds and suspicion of our neighbours across the Channel. For long periods, we were able to maintain a balance of power in Europe which served us well. Indeed, if we had not turned away from Europe in the Imperial heyday of the late 19th and early 20th centuries, it is even possible that the slaughter of two world wars might have been avoided. Are we now to isolate ourselves from Europe, at a time when our own strength is no longer self-sufficient and when the leading European countries are joining together to build a future of peace and progress, instead of wasting themselves in war? (...)

Accession to the Treaty of Rome would not involve a one-sided surrender of “sovereignty” on our part, but a pooling of sovereignty by all concerned, mainly in economic and social fields.

[1] Even in later studies on the foundation of the monetary union (the Euro) one of the explanations found for why the UK didn't join is differences in identities among elites, where Franco-german elites have incorporated the EU into their identities.

[2] In 1950, UK GDP per capita was 28% above the EU average, by the signing of the Rome Treaty in 1957 it was 15%, 10% in 1961 at the first EC application, 6% in 1967 during the second De Gaulle veto, 2% in 1969 and when they joined in 1973, it was 7% lower on average. All comparisons are with the original EU6.

[3] I can't find that much on the UK motivations in regard to EFTA, but considering they applied to the EU in 1961 after creating EFTA in 1960, it doesn't seem like the organisation not working was the issue. Rather EFTA was just formed at the same time as opinions were changing in the UK and the EC proved to do better than expected.

Sources:

Tömmel, Ingeborg (2014): The European Union - What it is and how it works London: Palgrave Macmillian

Nugent, Neill (2017): The Government and Politics of the European Union 8th ed. London: Palgrave Macmillian

Campos & Coricelli (eds. 2017) The Economics of EU-UK Relations - From the Treaty of Rome to the Vote for Brexit London: Palgrave Macmillian

Lord, Christopher (2018): The UK and European "centre formation" from 1950 to Brexit Geopolitics, History, and International Relations 10(1): 46-78