None of these add-on systems are as good as a well implemented factory system, despite what they tell you.
The FIAT group system involves the Code ECU recognising a changing code from the key, then sending a changing code the the Engine ECU, without this code, the Engine ECU simply refuses to function. The only way to "hot wire" it is to replace the Code ECU and Engine ECU (which are paired from new) with a set for which you have a valid key.
The best a 3rd party system can do here is something like block the signal between the 2 ECUs, but simply removing it will foil it.
Although Peugoet - at one point - had a cCode ECU which would simply supply 12V to the engine ECU if it is allowed to start, oops.
One major problem with that scenario in the OP's case, is that there is no factory fitted system in the world that will prevent teh third party from getting into and driving away the car if they have the correct set of keys.
The Clifford system mentioned by Dup is very good, also TOAD secirity do a fantastic system that allows you to send a text to your car which will immediately immobilise it, and do various other operations.
Autowatch is another system that has a good reputation, it operates every time you start the car, and if you are the correct driver then you will know the override, otherwise even if the thief knows the Autowatch system is fitted but does not know the correct override (which can be changed easily and regularly if needed) the doors will lock/unlock the siren and horn start blaring and the lights start flashing.
However to go back to the OP's question about legality, it is a VERY grey area. Currently, even the most recent Thatcham Category 5 systems, that are specified for high value vehicles, are only permitted to immobilise the starter circuit of a stolen vehicle via a secure operating centre only when authorised to do so by the Police and only then when they are within visual contact.
The legislation regarding such "after-theft" devices that immobilise vehicles automatically after the vehicle has been taken (whether when stationary etc or not) is a nightmare area legally in the UK, due to "Construction and Use" and "Road Traffic" regs and clearly does not meet the Cat 5 criteria or ACPO guidelines.
Additionally any electrical product that is intended for fitment to a motor vehicle must bear an "E" mark showing that it has been tested and approved for electrical compatibility (95/54 EC) for use in an automotive environment.
Most of types of "anti hi-jack" system aren't approved for European fitment (no E mark) as they were never intended for the UK market and were not officially imported (hence usually only marketed on foreign web pages).
So yes these sytems are available and can be used, but you are in a very very grey area as to whether they are legal or not and you may open a real mine field if you should ever actually need to use one and maybe cause an accident by using one.
