Hey OP, I'm guessing that you're asking about the British navy in the early 20th century, as you're curious about the two-power standard. I sent an alert to a couple of our 20th century experts to answer.

On the off chance you're curious about the English/British navy from say the 1580s on, I have some older answers that may be useful to you:

• How much more advanced was a British ship-of-the-line during the Age of Napoleon than a Spanish galleon in, say, the 1570s?

• Ship design and construction in the Age of Sail

• How would a British/HMS Frigate built in 1715 differ from his Napoleonic (say, 1790's+) descendant? What improvements would be made and did it's paint pattern or any aesthetics change?

• Did the Royal Navy build all its ships in Britain or were any of them built in the colonies?

• What advances in naval technology were made between Christopher Columbus and the American Revolution?

• What changes occurred in the construction, armament, management, and use of warships from the Anglo-Dutch wars up to and including the Napoleonic wars?

I have a question for u/jschooltiger regarding this post of his that was linked above.

Regarding the contemporary illustration of the Mary Rose, I noticed two cannons sticking out of the back of the forecastle which seem to point to the ship’s own mast. Were cannons actually placed this way, or is it just part of an artist’s picture? If so, what were they used for?

Britain was an island state, reliant for the past century on seaborne exports of manufactures and growing food imports that by the 1880s supplied nearly half of its food needs (even counting Ireland's surplus as domestic), and buoyed by an extensive overseas empire, all of which made seapower a vital concern. The sea also naturally offered a first line of defence against invasion, and control of key routes was necessary for deploying and supplying ground forces beyond Britain's shores as well as for purposes of blockade. As a maritime and imperial superpower committed to maintenance of a European balance, Britain faced a wide range of potential threats and demands, and required an exceptional range of functions from its fleet.

In terms of ships, guns or spending, Britain had already enjoyed a two-power capability at times before the Naval Defence Act (though the extraordinary mix of classes and designs of varying modernity makes a ship-for-ship comparison of fighting strength hazardous), most spectacularly in the decades after Trafalgar and most recently in the 1870s when France slashed naval outlays following crushing land defeat at Prussia's hands. Renewed French and to a lesser extent Russian activity in the late 70s and early 80s restored the balance only to roughly where it had been a couple of decades earlier, but provided a call to arms for the rising seapower lobby which could point to the undeclared two-power margin of safer times.

For the "how" it's worth recalling that Britain maintained a far smaller regular army than its most likely great-power adversaries, while enjoying a higher per capita national income than all except the USA. A £1 million first-class battleship of the 1890s or a £2m successor of the pre-WW1 Dreadnought period weighed in at less than a thousandth of national income or about 3% of annual defence spending. Overall naval spending was of course a good deal greater, but still rose only from under 1% of national income to less than 2% on the eve of war (though in its last phase the naval race provided a convenient argument for progressive shifts in domestic taxation that supported the beginnings of modern social welfare).

The truly remarkable feat is that even after rendering much of her existing lead obsolete with the Dreadnought concept, Britain retained a two-power margin in the new vessels almost until the outbreak of war, even then lagging by only one or two such units against an unlikely German-US combination (France and Russia having slipped down the rankings owing to a late start in the new race). Technical progress offered an opportunity for rival upstarts but could also be turned to the advantage of a seafaring power with the resources and determination to stay ahead: from the ironclad of the 1860s, the turret of the 1870s or the armour-piercing shell of the 1880s to the embrace of steel armour in the 1890s, the turbine and "all big gun" model in the 1900s or oil fuel in the 1910s, maintaining a lead involved a qualitative rather than a merely quantitative edge, a challenge for a conservative power like Britain but one that Admiralty chiefs and designers were able to meet after the obligatory early fumbling and fudging.

The standard treatments are Marder's Anatomy of British sea power and Kennedy's Rise and fall of British naval mastery, while Parkinson's The late Victorian Navy sheds valuable light on the international context of the 1870s-90s. Budget data are from Mitchell, Abstract of British historical statistics.