The nest system replaces both the thermostat and the programmer.
However the heatlink will probably not go in the same location that the programmer is in now, usually it will go next to the wiring centre in the airing cupboard.
Your thermostat will almost certainly be 230V currently, but the installer will connect this cable to the 12V terminals of the heatlink for the nest thermostat.
Sounds like I will be ok then , there is room in the airing cupboard for the heat link.