Pizza,
Actually not specific to U.K.
I have two alarms on my coupe. The first which is on my 300c, also, is just not as sensitive as it is on the 300. The reason is that the much shorter wheel base diminishes movement more than a long wheelbase like the 300.

Now the second alarm was installed to give me more sensitivity. It is marketed toward convertibles so that if anyone approaches the car it will warm them away. One of those.........if you can see it ........it can see you.
It has the voice options to tell you to "step away" and such but they recommend not using it as it attracts kids and others. Much better to send out sudden horrible sounds.
Since mine is a coupe, it still can see you, (your motion) through the windows.
So, it is like a super sensitive system since you don't even have to touch the car to set it off.
Of course I have to try to park where it is not possible to park next to me, but of course someone is always going to approach this car. It does warn in stages so that someone has options, like leave. Occasionally, I forget and get too close before hitting the fob.
I feel more secure with both types and of course would have to have it with a convertible, not wanting to up the top at every stop.

Phyrephylte