Teabag
The inputs to a Heatmiser UFH8 are 12v through Neostats - in the RF version you dont see it, but the actuators et al are switched at 12v with a 240v output.
For Nest, I assume you are switching at 240v with 240v outputs, hence using the Heatmiser UFH4?
My comment on not copying the UFH8 wiring across to the UFH4, but to work to the wiring diagram and “bell” the existing wires is good practice to avoid mistakes.
From your photos one of the valves is only taking a positive feed from the UFH8 - I assume that it works and that the installer was short of a core to bring neutral back to the UFH8 - that is not good practice. So, if it was me, I would check out all the wiring.
There is nothing wrong with what you are proposing with Nest and with the UFH4 it will work - apologies if I confused you.