Yet Another Nest v3 UK wiring and multi zone question

I’m very interested in using Nest v3 as it comes with HEatLink which can replace the Honeywell programmer currently attached to my boiler. However the only snag is I also have thermostat control at Ground Floor, 1st Floor and 2nd Floor. The current thermostats are battery powered but I’ve unscrewed the back of these and there are the two wires attached to it for, obviously, completing the circuit for the heating to start on that particular floor but there is also an unused, apparently neutral cable in the back box. So wiring wise I think everything is fine to be replaced by Nest - the issue is how does it all work together. I’ve drawn a rather rudimentary sketch of the current set up but essentially the Honeywell programmer and hot water control acts as the ‘master’ switch - if this is off it doesn’t matter what the thermostat on each floor wants (on or off) the Honeywell is master.

How woud this set up work replaced with Nests? I’m assuming the HeatLink clearly replaces the Honeywell Programmer and then will a Nest for each floor work - how easy would this be to manage from Home Assistant?

I realise this is an expensive unertaking - planning it as my Xmas present and hoping Black Friday offers something!