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The back of the rear seat can be lowered and items grabbed quickly. The side windows of cars and trucks can be broken in a similar manner with small portable tools designed for emergency rescue. A simple lever to open a locked trunk is available in many cars giving rapid access to the trunk.
You will probably find that your insurance is void unless the items were locked in the boot / trunk of a saloon car - I have had things stolen from the back of a locked estate car and they were NOT covered. This is the reason I don't even bother locking the doors of my soft top car - I would rather they open the door than slash the hood / top.
As the saying goes, over here across the pond in the U.S. we're separated from the U.K. by our common language. For the comfort and convenience of our American readers:British saloon car = American sedan https://jalopnik.com/this-is-why-we-call-cars-sedans-limousines-and-saloon-1825025822British estate car = American wagon or station wagon
...I suggest using an Airplane Mode if available. For example my Nikon D850 will communicate with my cell phone even when turned off. Laptop computers should be turned off. Some tablets will communicate with cell tower even when turned off. ...Dave Hartman