Why Britain helped the fleet receive a welcome in Japan

Was Japan allied with Britain when the Great White Fleet visited?

Yes. British diplomacy helped smooth the American reception, with an interest in preventing an expensive US-Japanese naval race.

Britain's alliance with Japan gave it a reason to help the American visit succeed. American diplomats organized the reception with British assistance. Friendship between the United States and Japan would reduce the pressure for competitive naval construction. Under the two-power standard, a race between them could also increase the number of ships Britain felt obliged to build.