As has been indicated above (irrespective of whether there are any such notices) the basic test in law is one of 'negligence'. If you break something in a shop through sheer carelessness the proprietor has the right to demand payment (and to pursue that demand through civil action if necessary).
Whether warning signs add anything to the standard legal position would depend upon the circumstances. As an analogy, a car park operator (such as a supermarket) can display signs stating that anyone parking there for over a certain period of time must pay a charge for doing so. By parking there, a driver has entered into a legally binding contract which is just as enforceable as a multi-page written contract signed in a solicitor's office.
Similarly, anyone entering a shop where "Break it and pay for it" signs are displayed could be held to have entered into a legally-binding contract by simply walking into the shop. However, if the shop owner left goods precariously stacked, a court might rule that such a contract was invalidated through the provisions of the Unfair Contract Terms Act 1997. (To use a further analogy, launderettes should not display signs stating "All washing carried out at owner's risk" because, for example, a faulty machine might give a hot wash when a cool one was selected and the launderette owner would still be liable to pay compensation to the customer if clothing became damaged).