ADT do have NSI accreditation for their hardware. I think they to lost it at one point, but theyve gone back to proper engineers doing the installations instead of salesman using sticky pads like Verisure, their sensors are "bank grade" 128bit encrypted and are frequency agile so are tricky to either block or spoof, etc. They were one of the firms our insurers found acceptable.
ADT also qualify for level 1 alarm response from the police, ie,they're considered trustworthy enough the dibble will attend on the basis of a call from ADT alone.
Verisure et al are level 2, the feds will only pitch up if they can confirm there is an intruder on site, and not just on the basis of an alarm activation.
Many local installers are indeed very good.
SSAIB is alternative body to NSI. Very few appear to have accreditation from both, but very few people have AA membership in addition to their RAC.