I am also in the UK and have extensively automated my house.
Like Robin, I use Fibaro modules behind the regular light switches. They allow me to keep the look of the house, are 100% compatible with the rest of the family and mean I can automate any existing light without having to change to expensive bulbs. The only snag can be the depth of the wall box behind the switch. You really need 48mm boxes or you will have to add a spacer. Schneider make an OK looking spacer that you can get from Screwfix. A great source for Fibaro and other HA items is vesternet.com. They carry a good range and the service is excellent.
I personally like Z-wave devices and for me they have been very reliable. Both Zigbee and Z-wave create a mesh to connect all the devices to the hub and the key to stable system is a good mesh. I have found that having a Z-wave module behind all the light switches in the house enables a strong Z-wave mesh.
I would not worry about the hub v3 unless you really want to connect it to the internet via WiFi. otherwise the v2 works well and you can probably find a bargain out there. Be advised though that HA is NOT plug and play. To add some devices to the SmartThings system you do need to add a device handler. Some of these are official, but most are written by folks on here. There are other systems out there that are far easier to setup. Hive particularly in the UK works well. However, you are restricted on what you can do. Once you get your head around The SmartThings app, developer portal (scary title, but its where you ad devices handlers and community apps) and how to add new devices its not so bad. Then you can start to play with things like routine, automation and Webcore. Thats where the fun really starts
All the best with the project. There is a wealth of info on here and don’t be afraid to ask if you don’t understand anything. We’ve all been there.