You need to make sure that you get Z wave on the EU frequency, and that you also get the smartthings hub on that frequency. That will make it much easier for you because those devices Will operate on the right current as well.
You can see all of the certified Z wave devices at the official zwave alliance site:
https://products.z-wavealliance.org/regions/1/categories
As far as where to purchase them from, Amazon.co.UK or any of the European Amazons are usually good.
Vesternet is excellent with excellent customer service and you may be able to get a discount if you are buying four or more of any one thing.
And there are additional retailers listed in the following FAQ. The ones marked “UK“ will usually be selling on the right frequency.
List of Home Automation Retailers
It’s true the echo has a zigbee hub inside it, but You won’t be using that because a zigbee device can only belong to one hub. Instead, you will be using the official “cloud to cloud“ integration between smartthings and echo.
With the hue bridge, it supports both ethernet and Zigbee. It will use zigbee to make its own mini network with the devices you attach to it. Then it will use your local ethernet network to talk to smartthings using the official integration. 
@anon36505037 is in the UK and has done to complete houses with Fibaro kit, so he may have more to add.