True, the British Army of 1870 is actually slightly smaller than in 1862 (the estimates started falling with the 1862 estimates, by 1870 much of the field logistics of the army have been dismantled).
The Prussian Army of 1862 is terrible, but we've already discussed that.
1) The British have a decent, well trained, modestly sized army at this time and a militia system, arsenals and industrial base to expand it quickly
Qualitatively the British are probably the best in the world, certainly they have the best infantry and artillery arms of any army. The cavalry arm is rather a blighted flower, its indulging in a period of woe and retreating into notions of fighting with breechloading and repeating carbines instead of sabres. They'll get over it though.
The British Army is a fairly decent size. At home there is a force of about 100,000 regulars. While smaller than the mobilisation strength of France (316,000), Prussia (212,000), Austria (298,000) or Russia (578,000) it does provide for a pretty potent force. On mobilisation the British Army, calling out it's Militia, Volunteers, Yeomanry and Reservists swells to over 390,000 at home. The strength of the British Army for home defence is the largest of any European nation except Russia. There are, of course, about 80,000 British soldiers in India and another 80,000 in the other colonies (notably in the Mediterranean), of whom a substancial disposable force can be culled (maybe 50,000 including deployable Indian troops). It is doubtful the British could project much more than 100,000 onto the Continent.
4) Confederate torpedoes (mines) only sank a few Union vessels, they won’t sink that many more British and French ones. They do consume lots of gunpowder.
The RN also had considerable experience of mine warfare, and of mine sweeping. Russian mine operations and RN counter mine doctrine is a lot more advanced than that of the ACW.
