David Harrision and Shaun come up with the goods regarding Thatcham. There were originally three Thatcham Categories:
Combined/Alarm Immobiliser
Immobiliser
Mechanical Devices.
There are now two other 'sub' categories: 2 to 1 upgrades
and post lost devices such as Tracker, Trakbak and Skynet etc.
For anything T2 or above, you are talking in the region of #200 absolute
minimum.
'Thatcham' is now widely acknowledged by the industry as the only standard and
any manufacturer that suggests otherwise has probably failed to meet the
standard themselves.
T3 devices are mechanical and consists of devices that lock up the steering,
such as crooklocks and other things that go through or cover the steering wheel.
I've actually use a 'stoplock' device on my mini, and you can pick this up for
around #50 in Halfords etc. In reality, T3 devices will only delay a determined
thief and obviously do not prevent items/accessories being stolen from your
vehicle.
Most insurers (the sensible ones anyway) use the Thatcham Standard as the bench
mark, although non-Thatcham devices are approved by some insurers due to
negotiations by the manufacturers direct with the insurer themselves (hence my
free stoplock above).
When writing high (theft) risk business, insurers will insist on T1 or T2 as a
minimum. When writing business I will also insist on a post lost device and
garaging on anything over #35k (and even security on the premises/garage on the
particularly high value vehicles) before I would even consider proceeding with
the risk.
Concerning other comments regarding theft etc., the vast majority of vehicle
thefts/attempted thefts in the UK occur in train station car parks. This is
because thieves know that commuter's vehicles will be left there all day and
lessen the risk of the thieves being disturbed whilst on the job (so to speak).
All commuter car parks suffer from this problem, but train stations have proved
to be the popular choice of all reputable thieves.
The earlier suggestion that thieves were more likely to vandalise the car in
situ if they couldn't get in going, as opposed to it stalling after a short
distance is flawed - think about it. Are they actually going to (noisily)
vandalise the car's outside your house with you inside or around the corner when
they are away from the scene of the crime?