"Founder liquidity" is worth looking up. If you're curious about this specific case, UK companies register accounts that can be read by anyone. I'm mostly calling this out as an example of why "public" is an important error in the op. Were the shares publicly tradable, the employees that chose to could have exited.
I'm not familiar at all with the UK law, but in the US the founders typically get common stock. They can't have it treated preferentially during acquisitions.
"Founders' stock" refers to the preferential tax treatment (TLDR: almost zero taxes via QSBS).
