Really?! Wow. It was very famous in the Sinclair community of the time. The last new computers from Sinclair Research were the QL (a 680xx home computer launched two weeks before the Mac) and the Spectrum 128, which used a design partly from Investronica in Spain, which tragically didn't incorporate the improvements from Timex in the USA -- it didn't have the extra graphics modes, which the Spectrum desperately needed, and it didn't have joystick ports or a cartridge slot. (Those appeared in the ZX Interface 2, of course.)

The two planned machines that Sinclair was talking about were the Pandora and Loki.

Pandora was to be a Z80-based laptop -- and bear in mind, at this time, laptops did not really exist yet. Sir Clive's second computer company did launch a derivative of this idea: the Cambridge Computers Z88. I have one. It's a lovely machine, hindered by nonstandard storage slots (because the standards didn't exist yet), and an awkward way of getting documents off the machine: RS-232 serial.

(The better way, IMHO, is what Alphasmart did later: make the portable appear as a keyboard, so it can just type your documents into any computer with the right keyboard port, whatever OS or apps you are running.)

The Z88 does show that Sinclair learned from his mistakes: it has its own proprietary OS, OZ, which is still in development -- a new version, 5.0, is in beta. https://cambridgez88.jira.com/wiki/spaces/OZ/overview

But on top of OZ it runs the standard Z80 edition of BBC BASIC.

The other machine, the Loki, was to be an all-singing all-dancing games machine, and it never happened in any form. https://en.wikipedia.org/wiki/Loki_(computer)