The key defensive problem is a rapid assault, not a months-long siege. A landing force might take a harbour or sandbar, but it would still need to break into fortified high ground while under fire from artillery, machine guns, mines, and light forces. The proposed defence can deliberately leave an inviting landing area exposed while keeping it pre-ranged by fortress guns. The garrison succeeds by denying the heights and surviving for days, not by defeating Germany indefinitely without relief.
Chapter · 54:11-1:00:48
Designing the fortress to defeat a rapid attack
The proposed British defence aims to survive a short German rush long enough for naval reinforcements, rather than endure an indefinite siege.