No harm in sticking an alarm on, but its not really worth putting a £200+ alarm on.
as it can be defeated as easy as a £30 ebay special.
It may go off for a couple seconds before its disarmed which may be enough for you to hear it and act.
But once they are in it takes mins to drive away.
I agree with Ratty
Id spend the £200+ on mechanical stuff instead of an alarm.
And and a £30 alarm as well just to make a bit of noise if you so wish
Your best bet is BIG YELLOW Mechanical devices. such as:
Disklok Titanium Silver Steering Wheel lock Small Size | eBay
CLUTCH CLAW LAND ROVER SECURITY MOTORHOME VAN CAR 4X4 | eBay
PedalLock - The second one completely enclouses the pedals so you cant get any tools behind it.
LAND ROVER DEFENDER SECURITY SAFE T PEDAL LOCKS FOR TD5 & TD5 XS LOWER DASH A/C | eBay
Adrenalin Defender Security Pedal Lock