I cant find anything about why you'd have this behaviour, (note the extra u in there for your benefit) but maybe this will help.
As I undertand it, the factory alarm in the UK models is different from the ones stateside, in that it includes a siren.
Maybe thats it.
-Blu
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Siren - An alarm siren is located on the front
extension of the right front wheel house panel in the
engine compartment of vehicles built for sale in certain
markets where it is required equipment. (Refer
to 8 - ELECTRICAL/VEHICLE THEFT SECURITY/
SIREN - DESCRIPTION).
ALARM - The VTA alarm output varies by the
version of the VTA with which the vehicle is
equipped. In all cases, the alarm provides both visual
and audible outputs; however, the time intervals of
these outputs vary by the requirements of the market
for which the vehicle is manufactured. In all
cases, the visual output will be a flashing on and off
of the exterior lamps. For vehicles equipped with the
North American or the ROW base version of the
VTA, the audible output will be a pulsing of the horn.
For vehicles with the ROW premium version of the
VTA, the audible output will be a cycling of the
alarm siren. See the owner’s manual in the vehicle
glove box for details of the alarm output requirements
of the specific market for which the vehicle
was manufactured. The inputs that will trigger the
alarm include the door ajar switches, the tailgate
ajar switch, the flip-up glass ajar switch, and in vehicles
built for certain markets where they are
required, the hood ajar switch and the Intrusion
Transceiver Module (ITM)
