The RN is more dominant in this period than in 1854,
This isn't true. In 1854 Britain had a greater preponderance in steam battleships over other nations than they did battleships in 1878, and a better doctrine for using them. By 1878 the RN, like all other navies of this time, is bereft of real tactics for a line action (even though none might occur, since the Russians will presumably hide in shallow water), is unprepared for even the slightest bit of commerce raiding, and has a motley assortment of battleships that have slow, inaccurate rates of fire and deep draughts, as seen by HMS
Sultan's performance against the
Huascar and HMS
Alexandra's grounding in the Dardanelles enroute to Constantinople in the 1878 crisis.
the Russian coastal forts are not going to stand any RN bombardment from the heavy rifled guns carried by RN warships.
There would be no need for a Crimean campaign for example, as the RN can reduce Sebastapol from the sea.
These are statements I'm leery of. Granted, the late 19th century featured some very nice examples of naval reduction of fortresses. However, it's practically never happened since, and there were actions such as Callao in 1866 where powerful ironclad forces were mangled by shore batteries.
This would all be moot if the Russian batteries are old and decrepit. Does anyone have any information on Russian sea defenses during this period?
A much larger British land force could be committed as well, for a short little colonial war with Egypt in 1882 the British landed 42,000 men (vs 28,000 landed in 1854).
Several thousand of those were, however, ships right off of battleships. These would be unavailable in a war with Russia. I'm nitpicking, though: your point is clear.
The British could immediately commit 70 or 80,000 men to a major war, a very large force by contemporary standards, and could rapidly ramp this up to over 200,000.
Hmmm...but these would be divided between a campaign in the Baltic and a campaign in the Balkans, correct? How many troops could Russia mobilize? I don't doubt that the Russians will be able to outnumber the British troops in any theater...should I? Of course, the Ottomans have to be thrown into the equation.