It is an offence to leave your car unattended, with the engine running, or incidentally with the headlights on.
The engine running stems from the war when enemies could make immediate use of transport if they found a running vehicle.
Some ecu's have a built in turbo timer you can activate, plod cars have "RUN LOCK" whereby the engine will run constantly but if you try to drive without deactivating run lock it cuts out.
tabetha