SmartThings is not well suited to a security system if you are in the UK unless you get the Vodafone V Home version. That’s because all the other models are cloud based (not just internet based—they also require that the SmartThings cloud be operational). All notifications, even just the ones to your own phone, require the cloud. And there is literally no way to arm/disarm the system if the cloud is not available, which can be really annoying if a siren is going off. Plus there is no locally functioning keypad.
I suggest instead that if you just want self monitoring that you look at Yale.
If you can wait, Amazon’s ring alarm is due in the UK later this year, and if it matches the US offering it will be a strong candidate. It offers optional professional monitoring at a quite low monthly cost.
Simplisafe is another low cost offering with optional monthly monitoring which is newly available in the UK but has been operating in the US for about 10 years.
Any of these would be more fit for purpose than smartthings, again except for the Vodafone model.
If you are looking for a Home Automation system, but not security, then smarthings is A top candidate for most people looking for low-cost systems. It’s just not well suited if your primary requirements are security, again with the exception of the Vodafone model, which is bought through Vodafone.