There are all sorts of very inexpensive ways of disabling your car (disabling the engine making it so that nobody could drive it even if they had your key) yourself without spending any more than a couple of £s.
Examples - Make a cut in the easily accessible (under bonnet) positive feed wire to petrol injectors / make a cut in the feed wire to the petrol pump. In these examples you could just fit a female 1/4inch jackplug socket in the wiring break and set it up so the engine won't run unless a male 1/4inch jackplug with shorted terminals is inserted (to electrically bridge the wiring break). I used to do exactly this on older cars (pre-1998) that didn't have chips in the ignition keys. You could even do this on wiring to key engine sensors such as the crank sensor. Thieves might know aftermarket off-the-shelf immobilisers but they could not anticipate such DIY immobilisation methods - which makes simple DIY immobilisers possibly more effective than expensive immobilisers!
But post 1998 cars have chips in the keys making it extremely difficult to start the engine without the key anyway. So if you want added security on top of the already very substantial security you could take it the other way and prevent a thief from being able to steer the car even in the (extremely unlikely) event they managed to get the engine to run without the key, could fit a steering lock and/or wheel lock.
The simplest way of disabling an old petrol car with sparks distributor was to remove the rotor arm from the distributor ;-) The only other ways would be to disrupt the negative/positive to the ignition coil but thieves with a bit of time might check for that... While even if they realised the car had a missing rotor arm they'd need the exact correct rotor arm to get the car running.
Depends on how far you want to take it and how great you think the risks are... Nissan made E51's with keyless entry and keyless ignition for convenience reasons, not quite so convenient when you have to insert a jackplug in a socket, remove steering wheel lock and wheel lock before you can drive away yourself