Hi and welcome.
I’m in the UK as well.
The zigbee frequency is universal so any zigbee that can pair with the ST hub, whether US or EU, will work to it.
Z-Wave is different. The frequencies between the US and EU are different, so you have to get Z wave devices that are UK compatible.
There is also a category here called “UK and Ireland Specific”, so that may be a good place to start.
I would suggest the devices probably do what they say, up to a point.
Contact sensor - Detects doors windows opening/closing.
Motion detector - detects motion.
Beyond that, see the following FAQ (this is a clickable link)
While zigbee is the same frequency worldwide, there are actually many zigbee devices that won’t work with SmartThings, and a number of US zigbee devices which would be illegal to operate in the UK because the US allows for higher power transmission levels. But see the FAQ above, it will explain further.