Thanks to Oakbank for starting this thread. I am not as familiar with weather compensation solutions as smart thermostats. In the UK at least they seem to be relatively rare/unknown/unpromoted.
However based on my knowledge and use of smart thermostats I thought it maybe helpful to detail how they relate to this issue.
The traditional boiler control method is/was a simple on/off control achieved by sending to the boiler a 'call for heat' signal. This causes the boiler to run at full power whilst the call for heat signal is present and then to fully turn off when it is not. This overall is a less efficient method than the newer approach which is referred to as modulated control. With modulated control the output of the boiler is varied depending on the demand and closeness to the desired heat level. Modulated control not only should achieve energy savings but help ensure a more stable temperature is achieved with less over and under shooting of the desired temperature.
It seems the weather compensation kit is causing a suitable boiler to operate in modulated mode even with a 'dumb' thermostat.
Since it is possible and arguably more common to achieve modulated control with a suitable smart thermostat this could be considered as an alternative way of achieving this and arguably has additional benefits.
In my case my boiler is located on a south facing wall with its exhaust also on that south facing wall. It would be harder to correctly fit a weather compensator since it would need shading from sunlight to avoid the temperature sensor reading being distorted. (You could fit a shield to prevent sunlight hitting it.)
A smart thermostat typically is able to obtain outdoor temperatures via an Internet service by looking up a local weather station and hence will not need to rely on your own outdoor sensor. This will therefore solve my problem.
A smart thermostat also adds the benefit of advanced scheduling and the ability to monitor the presence of occupants and therefore achieve additional energy savings. It can also link to a thermostat or even multiple thermostats and matching smart TRV valves and report to the boiler if any room needs heat. (If all rooms are reaching their desired temperature the boiler can then reduce its output.)
So whilst a weather compensator would and should be a definite improvement over the traditional call for heat method I would say a smart thermostat would be even better.
Note: There are two different but similar digital protocols which a smart thermostat can use to control a boiler to enable modulated control. There is eBUS and OpenTherm. OpenTherm was original developed by Honeywell and as its name suggests is now an open standard. eBUS is a proprietary standard - each manufacturer has their own flavour.
According to Tado a manufacturer of a smart thermostat and TRV system which supports both OpenTherm and eBUS they say eBUS is superior - mainly I believe in the area of fault monitoring of boilers.
Vaillant and Worcester Bosch only sell boilers in the UK with eBUS support.
Nest only has OpenTherm support.
Drayton Wiser only has OpenTherm support.
Most of Honeywells better smart products are US only but logically they also will only support OpenTherm.
As far as I can tell Hive and Netatmo do not support OpenTherm or eBUS.
As mentioned Tado supports both OpenTherm and eBUS - with the problem mentioned below for the newer Vaillant VR66 controller.
Due to local requirements in the Netherlands, both Vaillant and Worcester Bosch sell boilers fitted with eBUS to OpenTherm conversion modules _made by Vaillant or Worcester Bosch themselves_. However Vaillant at least will invalidate your warranty if you get and fit this genuine module in the UK.
Apparently Vaillant have also more recently changed/updated their eBUS protocol in the newer VR66 controller and not released the specs and Tado have not yet been able to reverse engineer this. The VR65 is supported by Tado.
There are some lesser known boiler brands which do have built-in OpenTherm support. See - https://myboiler.com/opentherm-capable-boilers/
The belief is that Vaillant and Worcester Bosch are using eBUS to mainly force customers in to buying their own matching compatible (semi) smart thermostats.