SHM is the SmartThings-designed alarm system, and is required if you’re going to use one of the paid monitoring services (which I don’t believe are available in the UK anyway). Smart Alarm is a community-created app that predates SHM by a couple years. Smart Alarm is now an official Smart App, so you don’t need to copy-paste any code. Smart Alarm has many features (entry delay, being the biggest one) that SHM doesn’t have. More info here: https://github.com/statusbits/smartthings/blob/master/SmartAlarm.md
To avoid cluttering up this thread, you can find more discussion about Smart Alarm here: