Also, a Halfords trolley jack dropped my car once when the valve just gave out. I wasn't even raising or lowering at the time, I was just standing there! The sound of the brake disc hitting the floor is not a nice sound!! Lucky I wasn't under it I guess.
Never leave a car resting on a jack, always put axle stands under the car and relieve the weight off the jack, though leave the jack just touching the jacking point.
To the op, if keeping in the boot look at low profile aluminium jack's, try Clarke from machine mart, sealer do them also and possibly others. The weight saving is worthwhile and will save your back and make it easier to strap down as the moving masses will be greatly reduced.