Why would Jellicoe choose the fast Agincourt as his flagship?

Why would Jellicoe take the speculative fast Agincourt as his flagship rather than assign it to the battlecruisers?

Its speed and Jellicoe's involvement in the design make it a natural fleet flagship, while its one-off characteristics argue against placing it in a homogeneous battle squadron.

Jellicoe would likely value Agincourt as more than a prestige flagship. Its speed would let the commander move ahead of the battle line when necessary. Jellicoe's involvement in developing the design would also make him well placed to bring a novel ship to Grand Fleet standards. Because battle squadrons were normally kept homogeneous, a flagship role is more plausible than attaching the one-off design to the Queen Elizabeths.