Bumping EC and NN's excellent points.
Simon's concern- "Well what kind of Japan is the UK in bed with?" is also quite valid, tho I think Taisho democracy would be a preferable partner.
For that to work, as EC said, Versailles had to work out better for the Japanese AND the Kanto earthquake have a better response.
It's a massive shame IMO that Taisho died as he did. He might've been learning disabled by modern lights but thoroughly xenophilic and progressive in his views.
He actually learned Korean and thought Koreans had a culture worth respecting. If he'd hung around a bit longer to really put his stamp on things, might have effected a much more benevolent Japanese rule over Korea or some kind of autonomy w/in the Japanese Empire, electing members to the Diet, etc.
Him living a decade longer/having a capable successor and the Diet getting enough of a capable civil government together MIGHT have butterflied Japan's slide into militarism.
An Anglo-Japanese alliance guaranteeing trade in the Pacific would help Japan prosper and develop economically as well.
I've banged on about how IOTL the Japanese military learned all the wrong lessons from the Russo-Japanese War and WWI that prompted their startling missteps in WWII.
They got used to fighting in backwaters against folks on the back foot up to that point and it made them feel a lot more froggy about taking on Western powers later on.
A significant butterfly would be the IJA participating in either the Eastern Front's war of maneuver or the meatgrinder in France would prompted a lot of internal rethinking re: banzai charges and elan substituting for proper logistics and firepower when fighting the Germans, Ottomans, and Austrians.
OR they could've emulated the Italian Army and fought hard, lost too many people needlessly, and veterans had even more reason to hate the folks in charge.
YMMDV.
Also, IF the IJA is heavily involved (100K+) troops on the Western Front, American, British, and European attitudes would change drastically about Japanese capabilities and the wisdom of keeping them onside, so hopefully, Versailles is less of a clusterfuck as far as Japan earning and getting their props.
IOTL the IJN did splendidly in WWI wrapping up the Pacific German cruiser squadrons and in support of various Allied ops in the Mediterranean, but that situation was well in hand as far as the British and french were concerned. IDK what else the IJN could've done- escort convoys across the Atlantic and done a heroic stand against U-boats trying to torpedo a liner or s/t to get some good press going.