I try to use the Venezuela Crisis with a UK more determined to go to war in order to preserve its position.
1. Like OTL, France and Russia ally in 1891-3.
2. Russia achieves strong growth (or seems to achieve strong growth), alarming UK and Germany.
3. An earlier Boxer Rebellion (1880s? 1890s?). France and Russia make gains in China, putting British interests there under pressure.
4. French-Russian alliance begins pressuring Britain on colonial fronts: Iran, Upper Nile etc.
5. France and Russia start beefing up their navies. Probably not necessary to comprehensively challenge UK naval superiority but just threatening key British routes (Suez etc.) will be enough.
6. Britain starts forming counter-alliance with Germany. To heighten the possibility of war, maybe Italy or Austria-Hungary defects from the Triple Alliance towards the Franco-Russian one.
7. In this charged atmosphere, US brings up Venezuela Crisis (maybe later than OTL - 1900s maybe?), and UK is faced with rollback of its influence on every continent.
8. Even here it's likely that UK concedes to USA rather than war with an extra enemy, so France and Russia have to do something to demonstrate that the Venezuela dispute will not just have regional but global implications for Britain's place in the world. Maybe the USA seeks support from France-Russia, or some 'USS Maine' event which can stoke war fever in the US.
9. UK and US go to war, France-Russia intervenes, UK goes to war with them as well and everybody drags in all their allies.