What should be ring fenced are services. A government, any government, decides what its priorities are and allocates the necessary funds to those services. There's no point in ring fencing a budget the size of health or education. You have to allocate resources to priorities within those vast budgets and make tough choices about what will have to be cut once those priorities are provided for.
That is the way to cut spending. Start with a clean sheet of paper. Decide what you want to do and what is less important and cut up the spending cake. Simples as everyone seems to be saying nowadays.