We should add that SmartThings staff have now confirmed that the newest versions of the SmartThings-branded zigbee motion and multi sensors (but not the water leak sensor) should operate within the UK legal limits.
However, Devices that do not carry the CE electronic certification might not be allowed for import, so you do run the risk that they could be seized at the border if they don’t have an EU safety certification. So you need to check the label on each device.