For what it is worth the two biggest names for boilers in the UK (Worcester Bosch and Vaillant) use their own communication protocols and mostly do not support OpenTherm (there are some third party products that act as intermediary devices).
The key benefits of OpenTherm would be Modulation (basically a dimmer switch for the boiler) and two-way communication (eg. Set target temp from thermostat, receive current water temp from boiler).
Given the above, I personally opted to stick with Worcester Bosch as I am not a plumber and wanted something I know it would be easy to find plumbers to work on in the future.
As for Thermostat, I opted for Hive which I have paired via Zigbee2mqtt. I would add that the Thermostat receiver paired easily whilst the thermostat remote control was a little finicky and I had to try a few times.
I did not try the Hive Hub or the Hive App - just stuck to Home Assistant with Zigbee2mqtt.
I have several TRV's that control the radiators as well as Air Conditioners which can be used for both Heating and Cooling, so I rely on custom scripts to determine when the the boiler should be providing heat. I trigger the boiler by setting the Hive thermostat to either a high value or low value depending on whether or not heat is required by any of the radiators.
I would add that I only control the boiler for Heating although it does provide hot water as well. So, in essence the Hive Thermostat is basically an On/Off switch for the boiler.
One of the key things in my decision, was the fact that if I need to move, I can leave the Hive system in place for the new resident being able to use it via the Hive Hub (and whilst I am not using it I did get a Hub).