On my suggestion for a simple test, remember the differential head from highest faucet to basement is not much, so any gage range used has to reflect that. Each foot of water head loss is only roughly about 0.4 psi. Another way to do this test is with a clear plastic hose attached to the drain in your water heater (assuming it is in basement). Extend that open end up above the height of the highest faucet outlet and fill it with water until it is at the level of the highest faucet or shower head. Then shut that off. You don't open any valves until curb valve is shut off.
Then starting with the shower head, open that one and then the basement hose valve. Then you have a measure of the pressure head in your system at the basement by viewing that hose water height. There may be a clear plastic garden hose that you can use also. If seeing water is difficult, add some food colored water in the upper end at the start. You need to be able to view the pressure head in that hose. For a typical house and with the rate of flow you observe, this test should not take more than an hour, possibly a few minutes.
For the curb valve (at least in USA) the cap for the valve is held a five sided bolt which wrench is obtained at any plumbing supply place. The valve key wrench is nothing more than a long rod with "T" Handle on top and a forked bottom end. Most any plumber would have one. The depth to the valve top may vary, maybe 4 feet or maybe 8. The handle orientation is the direction of flow. A 90 degree turn shuts it off.
Now comes a possible problem. Suppose you don't find any head loss. Either it is ground water or the leak is beyond that outside valve. The test would only test the water line between your house and that curb shut off. It is possible for the water main in the street to be leaking near your property. I have seen two situations where that valve has had corrosion and has leaked badly, but not on the side towards the house.
On the subject of ground water has any geologist type looked at the area geology? For instance, some land forms are prone to have springs and others are not. Glacial country does have them, but a geotech eng'r or geologist should help on that score. Are you on a side hill or near a hill? You are right, it would seem that during construction they would find the steady ground water flow if that is the cause.
Is there any possibility of some day draining that perimeter system to daylight or a street storm drain, say at a back yard?
Cracks in basement floor would only be likely from this water thing if there had been erosion via the tile drain system. For a system that is not properly filtered this can be a major problem. If you pound on the floor with a big hammer does it sound hollow? Remember, concrete sometimes curls and cracks so it might be hollow at the cracks then, not related to erosion. You can observe for curling by placing a straght edge across the cracks.