Why Amagi would not simply replace the Royal Navy's N3 program

Would the Royal Navy choose repeat G3 battlecruisers instead of N3 battleships after seeing the Amagi class?

Probably not, because the N3s would already be underway before the Amagis completed; the Japanese ships would influence a later British generation instead.

Projected order and completion dates matter more here than a comparison of completed ships. The G3s were ordered in 1921, the N3s would likely follow roughly a year later, and both British types would be well advanced by the time the Amagis entered service in 1923-25. Britain therefore could not simply substitute repeat G3s for N3s in reaction to completed Japanese ships. The Amagis would instead inform the next Royal Navy design generation. That later choice would weigh foreign construction alongside new technology, improved imperial infrastructure, and the service's preference to present itself as the benchmark navy.