That looks like the HMS Polyphemus, laid down in 1881, and featured quite a lot in the UK naval maneuvers of 1886. See the 1886 Brassey's Naval Annual. She was coal-fired, with a top speed of 18 knots, which matches the description, and did carry five 14 inch torpedo tubes, including one in the bow that was covered by the ram. To fire that torpedo tube required the ram to be lifted out of the way. Based on D. K. Brown's book, Warrior to Dreadnought, the 14 inch torpedo carried a 26 pound charge of guncotton, had a range of 600 yards, and a speed of 18 knots.
By the way, 3 inches is 76.2 millimeters, while 47 millimeters is the standard 3 pounder caliber of the period, meaning that the standard shell weighed 3 pounds, and the caliber in English units was 1.85 inches.
The armor was a bit unusual, being composed of two layers of plates, one quite hard while the other was tougher. Each layer was 1 inch thick.
The ship did cost £226,000 pounds, about £100 pounds per ton, and was viewed as a fairly good deal. In 1886, 18 knots was viewed as adequately fast. She definitely was a more useful ship, as a ram, than the previous rams purchased by the British Navy.
There is also a good write up on the "Polyphemus" in Admiral King's The Warships and Navies of the World 1880, which has been reprinted by the U.S. Naval Institute Press. This write-up gives the design specifications along with some of the justification for the vessel.