Worth noting that NSI is not the only accreditation body out there in the alarm industry. There are a lot of smaller SSAIB approved alarm companies who often tend to be slightly cheaper than NSI approved firms. SSAIB are also recognised by most UK insurers. See SSAIB directory - https://ssaib.org/security-provider-search/ Those who mention ADT, it's worth noting their full costings, often not published online but available here. Note: the extra devices you add to the system also increases your monthly fee. Those who are interested in Verisure intruder alarms, the website above also has a special offer (?500 off) and appears to be available in the London area (East Dulwich included), Verisure offer. Heard of Banham Alarms, but don't see much about them online.
