The Japanese government's first priority in foreign negotiations was the United States, but the second priority was the United Kingdom. This was due in part to the fact that the UK was the largest negotiating partner for importing the resources needed to run Japan's economy.
Great Britain.
Japan needed to import a variety of things, notably iron resources from Australia and crude oil from the Middle East. The problem was the price. This was different from the relationship between Japan and the US, which was based on the American military presence in Japan, so it was a difficult task. The United Kingdom, whose national strength had been weakened by the end of the World War, wanted cash, but the Japanese government was reluctant to make a deal with easy money because it would leave a bad impression on the future.
After nearly six months of negotiations, the Japanese government decided to follow the precedent set by the People's Republic of China. It offered to pay the British for major infrastructure investment. It proposed a variety of things, from state-of-the-art infrastructure on the British mainland, to the expansion of the Suez Canal, to oil and freshwater refining plants in the Middle East, to port facilities in Australia.
In a sense, it was a proposal for investment that could contribute to the revitalization of the stagnant British economy. Moreover, Japan would be able to mediate work to the domestic general contractors and their surrounding companies. It was a win-win proposition.
The United Kingdom agreed to take advantage of this opportunity. Since time was needed to develop the infrastructure, the amount of investment and the amount of imports between Japan and Britain were to be formulated over a period of five years. *1
There were two other issues in the negotiations between Japan and Britain besides trade. One was the Anglo-Japanese Alliance, and the other was the League of Nations.
Japan's membership in the League of Nations and its permanent seat on the Council of Nations were replaced without problems, as Japan declared that it would take over the treaties and debts that the Japanese Empire had concluded with other countries. The problem was the treatment of the Anglo-Japanese Alliance. Due to the complexity of the issue, it was shelved after the trade negotiations.
At the same time, the issue of the Washington Naval Treaty became a major topic of discussion. Japan did not have any battleships, but it did have one aircraft carrier, and the second was in the outfitting stage. It was a large ship (from the British perspective) with a standard displacement of 62.000 tons. With two of these ships, it would have far surpassed the Washington Naval Treaty that the Japanese Empire had signed, and it would have also exceeded the standard displacement per ship. In addition, the mainstay destroyers also had to meet the maximum displacement, and the total displacement was also in excess. By the common sense of the era, all of the destroyers in Japan's fleet were large ships like light cruisers.
The United Kingdom and the US, the main parties to the disarmament treaty, were naturally puzzled by this. As a precaution, they proposed to Japan that they disarm in line with the treaty, but they refused. On the contrary, Japan proposed that the destroyer be registered as cruisers.
With the cruiser quota, there would be no problem if they turned a blind eye to the guns and appropriately divided them into A (heavy cruiser) and B (light cruiser) categories. At one time, there was a belief that this would be fine, but there are 22 FFMs (frigates) not included in that category. These were also 3.900 tons by standard, well over the maximum quota for destroyers. The submarines were well within the total displacement of the fleet. The submarines were well within the upper limit, though.
They can't use today's common sense to plan for the armaments of a century from now. That was the conclusion of the British and the Americans.
At the same time, however, the two countries saw in the negotiations that this Japan was an extremely moderate nation compared to the Japanese Empire and would not expand its military excessively. The fact that, coincidentally, both countries were able to hold on to Japan's two lifelines of food and resources was also a relief to Japan. *2
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Notes
*1: The first Japan-Britain Treaty of Commerce and Investment was concluded. The results of the first treaty were to be considered for the next round of trade. The first treaty was in effect from 1926 to 1931. The second treaty was terminated after three years, from 1931 to 1933, and was renewed for a third treaty. This was due to the establishment of Nazi Germany, and a shift in the direction of war preparations. For this reason, the Third Treaty included not only infrastructure development but also the provision of war equipment to the United Kingdom. In addition, the Anglo-Japanese Alliance was also revised to promote cooperation in the defense of the British mainland.
*2: This would eventually lead to the 1927 Japan-Britain-United States Agreement of the Limitation of Military Power in the Pacific Rim. However, before the agreement could go into effect, France became involved and objected, so it never went into effect.