I think you can still get there. The Germans were very upset the USA and others kept resupplying the French. Adjust the land victories enough to keep the French in the war longer. Supplies become an issue, and at some point, the UK/Prussia relations deteriorate enough to get the UK in the war.
There would be a problem: Germany did not have a navy and UK did not have an army.
But this is quite irrelevant because your idea about pro-French Britain does not have traction with OTL reality:
"Britain viewed France as a serious threat to their position in Europe and wished to make sure France would be weakened at all costs. This view of French hostility from the British was solidified at the time due to the competition amongst both nations to secure and develop successful colonies. This often pitted the two nations against each other and created antagonistic economic policies toward one another. At the same time, Britain had natural inclinations to want to ally with Prussia due to their close religious roots and common rivalry with France. There had never really been much strife between the two countries prior to the early 20th century and the First World War. It happened to be that Britain and Prussia’s objectives lined up from 1870- 1871. Britain wished France to weaken as a nation within Europe to help maintain their dominance over the continent and Prussia wished to defeat France in a war they were confident in achieving victory in to help unify all of Germany under Prussia leadership. For these reasons, the ground was set for a natural alliance between Britain and Prussia and a mutual enemy of France.... After the war had been declared, a resounding amount of newspapers viciously blamed France for the war.... Several members of the House of Lords condemned these French actions."
https://vc.bridgew.edu/cgi/viewcont...httpsredir=1&article=1125&context=honors_proj
There was also a recent (1869) crisis between Britain and France over the issue of the railroad concessions in Beligium during which Prussia reached to Britain offering a joined defense of Belgium in the case of French attack which contributed to the pro-Prussian sympathies in Britain and the sentiment did not change during the war.
As far as Britain was involved, the only question was a degree of Prussia's/Germany support which varied from unconditional enthusiasm (“The Queen [of England] rejoiced at the triumph of Germany. She completely identified herself with the German People.”) to a slightly more cautious views about degree to which Germany should be supported. But this belongs to post-defeat period when negotiations of peace began and, as Gladstone put it, France got some sympathy "because she was the worsted power". The idea, however, was not to come on the French side but just to limit size of the reparations and territorial losses.
If we are looking at war going into the second year, it is easy to work the butterflies to get the others in the war. Maybe then someone see a chance to take care of an issue while the UK/French/Germans are distracted. Russia wants to force the straights. Italians and/or Austrians see a chance to gain something, somewhere.
More of a fantasy unrelated to life. In 1870 Russia hardly completed its military reform by 1877 and in 1870 simply was not, economically and militarily, in a position to start a major war.
