I'd never heard of the Thatcham ratings. Maybe some other folk have not either. I found this link helpful:
Thatcham Category 1
Find out what thatcham category car alarms are inlcuding Thatcham Category 1 and 2 alarms and how they work for car security systems.
(quoted from the above website):
Systems that fall into this class are the cleverest and most complex on the market.
A Thatcham Category 1 alarm will feature perimeter and ignition detection, and incorporate movement, glass break and/or tilt sensors. There’ll also be a siren powered by its own battery supply that will sound if your car gets broken into.
Immobilisers are also a requirement to pass Thatcham Category 1 tests and have to be passively set – that means without any action from the driver – while a minimum of two operating systems or one control unit used for normal operation must be isolated.