A belligerent Germany is probably the only thing that could fulfill the need of "On the scale of the Napoleonic Wars" though, or else we have a Crimean War Part 2? That's just what I think though. Germany in the alliance with Britain seems logical too, though I don't know how the French would react. But now Russia is doomed to an extremely humiliating defeat.
What we need is to expand the scope of the conflict, otherwise London's side of the conflict has it way too easy. Maybe the Russian commander in Turkestan decides on his own initiative to intervene in the Second Anglo-Afghan War. Russian commanders in Central Asia often did reckless shit like that; you could have General Kaufman die early and replace him with Mikhail Chernaev, his OTL replacement and a known loose cannon. Ayub Khan gave the British serious trouble on his own, and Russian support could make him too tough to handle without more troops than the British could spare. Regardless, Russia gets dragged into a war with Britain more or less against the Tsar's will. And considering the Tories are in power, London won't back down. Some prior POD work could have this coincide with war in the Sudan and maybe even another Sepoy mutiny, if Roberts' army gets massacred.
In the meantime, Russian diplomacy buys Austro-German passivity while they take another crack at Constantinople - in for a penny, in for a pound, no? That pulls in the Turks and possibly the French. The war goes badly enough for the Turks and their allies that Austria-Hungary and Prussia decide to intervene in Russia's favor, since they seem to be winning. That gives you a war involving everyone but Italy, and London could get them in by offering Trieste or something. It's also much less one-sided than the Crimean War, so it'll be sure to surpass the Napoleonic Wars in scale.