Successive minesweeping and anti-submarine batches gave Britain and France 120 Flower-family sloops, about 100 of which survived the war and immediate aftermath.
Treasury pressure shadowed HMS Enterprise's completion, while the sharper Royal Navy debate concerned the cancelled third E-class cruiser, HMS Euphrates.
About two World War I Flower-family sloops reached World War II service, with HMS Foxglove surviving German air attack and remaining useful in a support role.
A bomb amidships could destroy machinery spaces, while comparable hits near the ends of a sloop or destroyer left more structure available to contain damage.
HMS Veronica shows how inexpensive station sloops extended British presence while helping Dominion navies such as New Zealand's build local capability.
Home and Mediterranean sloop service created mine and anti-submarine specialists, with Frederic John Walker as the best-known product of that professional stream.
A continuing sloop program would not replace destroyers, but it could accelerate dual-purpose guns and enlarge the turbine-production base shared with wartime construction.
The demanding Perisher course selects submarine commanders, but a cheaper diesel-electric boat could provide independent sea-command experience before an officer receives an SSN.
A sloop accompanied HMS Birmingham to recover SS Vincent de Paul, adding presence and tactical flexibility without making the force look needlessly escalatory.
Sloops sacrificed destroyer speed and torpedoes to become adaptable minesweepers, escorts, net vessels, anti-aircraft supports, survey ships, and persistent local callers.
Speed, roughly month-long endurance, fast boats, a helicopter-capable deck, and space for up to 50 embarked personnel make Batch 2 Rivers useful for persistent security work.
A 57 mm gun offers a real capability increase over 40 mm, but its larger cost jump makes an OPV look more like a cheap frigate without delivering Type 31 capability.
Thailand can justify a 76 mm OPV gun because it is common across its fleet, while Britain gains a cheaper and more proportionate upgrade by standardizing on 40 mm.
Twenty-millimetre guns remain common on NATO OPVs because they are cheap and proportionate to many small patrol ships' operating areas and likely threats.
Danish modularity makes the Knud Rasmussen-class patrol vessels a compelling modern sloop model, but another navy must fund modules, support, and common interfaces across more than one OPV class.
If Type 31s are late, extending suitable Type 23s preserves real frigate capability more effectively than taking River OPVs offline for complex upgrades.
BAE's Avenger proposal showed how a River hull could become a corvette, but the result did not create enough capability beyond an OPV to justify choosing it as the Type 31 frigate.
Britain could supplement scarce SSNs with cheaper SSKs or build two extra Dreadnought-family hulls able to alternate between guided-missile and ballistic-missile duties.
Turning on the shipyard pipeline too quickly creates empty hulls unless recruitment and training expand early enough to provide crews when the ships commission.