Can you install a storm porch AND NOT digging down the base for the storm porch

When a structural engineer carries out calculations he assumes the worse; this, in your case will include the weight of the brick, UPVC frame and glass, roof and any adverse weather conditions such as snow which will sit on the roof. In addition to the weight of the building you will be standing in it when you use it along with any thing you decide to store in it. Your flags are probably 2 inches thick and if they were 'laid correctly' are sat on compacted sand and several inches of hardcore underneath. If for example you had a leaking gutter that started to wash the sand away under the flags; your porch would start to move and come away from the building If you had proper concrete foundations that would not happen Ultimately the choice is yours if you do it yourself; if someone else does at your behest then you are unlikely to get a very professional job and will almost certainly not be insured when the inevitable happens