Why not build 25-knot fast battleships from the start?

Why did the Royal Navy build battleships and battlecruisers instead of making all its dreadnoughts 25-knot fast battleships?

Contemporary hull, armor, subdivision, boiler, and machinery limits made that balanced speed expensive and oversized, forcing designers to trade armor coverage against speed and firepower.

A heavily armed ship needs enough internal volume for armor protection, subdivision, magazines, machinery, and boilers. Reaching twenty-five knots with contemporary technology while retaining battleship protection would produce a larger and more expensive hull than the navy was prepared to buy. The design balances speed, armor, and firepower within that constraint. In the distinction made here, fast battleships accept somewhat less armor coverage for speed, while battlecruisers accept further reductions in coverage and subdivision. The separate dreadnought armored-cruiser concept takes those reductions to cruiser standards. HMS Tiger complicates later labels: her twenty-eight knots and armament comparable to contemporary battleships accompany armor coverage and subdivision judged favorably against Hood for her size. Calling Hood a fast battleship would therefore require reconsidering Tiger on the same criteria. Hood began from a more Fisher-influenced concept, while the G3 design more clearly continued the direction associated with Tiger. The categories depend on the technology and compromises of their period.