I agree that if you had a bloody minded copper you could be charged with having an unsecure load, but as the piece of equipment isn't actually a loose peice of goods being carried IE a suitcase on a roof rack then it would be un-likely to succeed at court.
I see daily proffessional drivers carrying goods that clearly are being carried far more dangerously than any roof rack load.
For example in my area we see curtainside artic lorries carrying wood chip loads of about 25tonnes, relying upon side curtains clearly bulging more than 300mm from the side structure of the trailer, but they never get prosecuted. All they rely on is the strength of pvc tarp and straps designed primarily to contain the curtains not to restrain the load. Should some yob slash the curtain, then there would be a zipper effect tearing of the whole curtain and ok, the yob gets dumped on, but who else might get caught.
As I've said already as far as security of the caravan and it's fittings are concerned as long as you have checked and satisfied your self that at the start of your journey all lights work, sky lights are closed and firm, windows closed and secure, wheels and tyres are ok, caravan is fixed to the car correctly etc etc, then basically you have done your duty of care and should anything happen after that, that you don't notice or expect, it is purely an accident.
Steve L.