Morris 1100 wrote:
A mini with an immobiliser would take less than 30 seconds to start

Bit of a blanket statement, isn't it? For example, the transmission lock / immobiliser on a Saab is pretty much impossible to break. The electronic "keys" that cars come fitted with as standard are beyond 99.9% of thieves, and there are retrofitted kits that use a reception loop around the ignition lock that require a transmitter on your key chain to unlock the electrical system. A recent issue of Practical Classics mag detailed how to fit it, including all the black wiring and so on.
So, if you want to protect your car as best as you can, there are products available out there, right up to those flamethowers fitted under the car to prevent hijacking. Even if they aren't legal or morally acceptable.
I just use one of those locks that go over the handbrake and lock it to the gear lever, but then it's a slightly rusty Mk II 850 so who would want to nick that? I don't even lock the Commer.