Step one is read
the installation instructions what we are looking for is can a modulating thermostat be fitted?
It would seem with your boiler there is no provision for using a modulating thermostat, the Nest can be used as a modulating thermostat with boilers using OpenTherm but in your case there seems no option to use modulating thermostats.
So with your boiler only option is the use the thermostatic radiator valve (TRV) which will modulate the boiler, you can improve these devices by swapping the heads for electronic versions. However now we reach the next hurdle.
Without some hub which connects to the boiler, the eTRV will work A1 in heart of Winter, but as Summer them closing with cause the bypass valve to open and the boiler will start to cycle, you could of course manually turn off the boiler, but if you want automation then ideal is for some device to collect information from the eTRV heads and when non are asking for heat for it to switch off boiler.
In your case likely two options.
1) Fit EvoHome very expensive but will do the job A1.
2) Use TRV heads which will work with Nest.
I will look at option 2) as far cheaper, at least for you with Nest already installed, the MiHome Energenie heads are designed to either follow Nest or Nest can follow them, so using IFTTT you can fit the heads and all rooms with the new heads will be at same temperature as the Nest is set, it has a disadvantage at the moment over EvoHome in that you can't have each room at different temperature if following Nest.
I only have the eTRV heads fitted, and they are good at regulating the temperature, the main down side is they have very good anti hysteresis software, so set from 16°C at 6 am to 20°C it can be 11 am before new temperature is reached, I cheat and set to 24°C at 6 am and then back down to 20°C at 8 am and this works fine, remember I don't have Nest, likely they will work better with Nest.
One thing they do have is two temperature sensors, so the air temperature reading is compensated by the water temperature reading so they are not affected by the heat from the radiator they control, so set at 20°C the room does reach 20°C.
However still not what I would call cheap, you need a hub as well as the heads, a cheaper way is to use self contained units, however since you already have Nest I would think worth the extra to have valves which work with Nest.
EvoHome is better, and if you latter want to go to EvoHome then you would not use Energenie TRV heads, so you need to look at price and decide if worth it.
