Estimated strength of U-profile shelf standards?
While they may work for a table that is shallow, not cantilevered off of the wall very far, it really is not the appropriate hardware for the job. Assuming you fasten the shelving standards to the brick wall with the appropriate fasteners, i feel the week link in this plan is the little tabs on the back of the bracket that hold the bracket to the standard.
I would invest in these brackets.
I have used them to make desks, they are great and come in a variety of size's to accommodate shallow or deep desk/workbench's.
Here is a link to a desk question that I answered that shows the brackets in use.