Daring-class plans would give 1934 Britain technology it could actually build

Which modern warship blueprint would be most useful in 1934?

The 1950s Daring class, because its weapons, radar, machinery, and electrical systems were advanced yet buildable with 1930s industry.

Daring is close enough to contemporary manufacturing that Britain could reproduce its 4.5-inch mounts, radar, torpedo launchers, machinery, and electrical lessons. The class's split AC and DC installations would also reveal which shipboard power standard proved superior, accelerating a broad family of practical improvements.