IANAL, and certainly not British either, but as a matter of precision, the conviction rates are fine, and probably quite high. Successful criminals would not be caught, though theft might or might not be reported depending on how the inventory is handled and shrinkage is accounted for. Unsuccessful ones would still be caught, and in general being caught shop lifting, but not via camera, means you have the stolen property on your person. Which makes convinction guaranteed. Now the numbers of conviction, numerically, to the shoplifting events might be low due to decision not to prosecute, but the rates of conviction to prosecution for the crime would be as high as ever unless they start pinching people long after they've gone home, without evidence or eye witness accounts.