Every project that never made production is touted as being wonderful, and only the machinations of politicians/ministry/Beelzebub stopped it being a world-beater.

Quite. 🙂
Large aircraft of the 30s (and if we’re giving Mitchell credit for the design it would have been mid-30s given his death in mid-1937) were inevitably underpowered, you have a great airframe but without reliable and powerful engines, it’s not going anywhere.

And let’s remember even though the designer may be great, every design may not be.
Case in point: the Tudor.