Openhab is an open Source home automation platform with a very wide range of integrations that can run on a Raspberry Pi. HomeAssistant is another popular hub for home automation.
Is my protocol of choice for sensors - currently most of those are Xiaomi based but I am about to recieve a USB Zigbee endpoint and am starting to find other budget priced Zigbee devices. I'm now using Zigbee2MQTT and USB Zigbee Controllers in multiple locations feeding back though a VPN to an MQTT Hub running on my Openhab Instance.
I'm also using a range of wifi based devices
Range of cheap Zigbee based devices including a Power Socket for £10/€8
Cheap Temperature & Humitidy sensors, Door & window sensors, Infrared controllers, Power Sockets and other devices mostly based on the Zigbee protocol (but in a slightly proprietrary way ). They use their own hub that itself can be integrate with OpenHab. These are all available to purchase from places like Aliexpress.
I'm naturally staying away from the likes of Hive and Next. I don't want to be tied into their services and providing my data to them. Instead I have been looking around for Zigbee based Combi boiler thermostats. They are on the market but I'm looking for one with a very simple Physical Inteface similiar to the traditional Honywell rotary thermostat. I have found some - but they are eithier available from Taiwanese manufacturers or use OpenTherm to communicate with the bolier control unit.
Indeed the Heating control space seems to be dominated by proprietary protocols or those specific to those types of Hardware
Like Zigbee & Inter
The UK's rollout of Smart Energy Monitors has been a complete mess (and expensive to). I'm looking a low cost whole of house energy monitoring solution that is focused on collecting the data and making it available to an open home automation system rathe than trying to create its own eco-system with a tied in Cloud service and App.
The closest I have found so far is the Aeotec Clamp Power Meter
I have also had a look at Bulb and Octapus Energy - but both seem to be going down the web services route where data is provided back from the retailer rather than directly from the SMETS metering equipment.
http://www.climax.com.tw/new/clmt-1zbs.php - More than I was hoping to spend and Zwave not Zigbee
https://www.develcoproducts.com/products/meter-interfaces/external-meter... - But no clear place to by
https://www.neur.io/energy-monitor/ - An integrated solution. Simple installation and I like what they have done with their app but no doubt a closed solution and no reseller in the UK.