That diagram seems to show both a neutral and a ground. Both exist in the UK? Don't they use a delta configuration for most of EU to save copper? Are both supplied by the sub-station?
Yes. Neutral and ground are separate.
No. Delta is not used at low voltage. The final service voltage is 230/400V wye. In Uk, most residential is single phase 100A @ 230V. Large residences and small commercial premises will usually have 3 phase 100A @ 230/400V.
Whether both neutral and ground are supplied by the sub-station depends on the type of service and cabling.
TN-S (separate earth supplied by substation), TN-C-S (combined neutral/earth supplied by substation, with redundant earthing in transit and separated at point of service drop) and TT (no earth connection provided by substation, customer provides earth rod at service entrance) are all common.
So what does your electrical code specify for the maximum continuous load on a 40 amp circuit?
Maximum continuous load on a 40A circuit is 40 A. No derating is required for load duration (doesn't stop it being a good idea), and in general is not performed. For example, no specific derating is needed for a 3.1 kW storage heater which is designed to take full load for 7 hours to charge it's thermal store, and the closest circuit capacity available (16 A) can be used. Typical installation cable is 1.5 mm2 flex (for final connection) and 2.5 mm2 in-wall wiring (derated due to thermal insulation).
Derating is performed based upon ambient temperature, cable grouping, cable mounting surface and insulation.