As far as I can make out, there is no way for current speed to be displayed. The system can be made to beep if the speed limit is exceeded and the speed limit is displayed (on faster roads) but not the car's actual speed.
Most trafficlaw enforcement agents use radar and not satnav readings to determine the speed of the subject. So it is not advisable to use the satnav as the means of determining one's speed. Furthermore, satnavs have an inbuilt error, as they do not resolve down to a fine detail; only to a few metres. One can see this if one deviates from the course chosen by the satnav. It takes several metres of deviation until the satnav snaps back onto the actual road upon which the car is travelling. The error is typically 3 metres.
Another error with satnav is the vertical distance. Travelling up or down hill will introduce further errors in the satnav speed reading.
Speedometers are designed to never read less than the actual speed, and are permitted to read up to 110% of the actual speed + or - 6mph.
Ran a test the other day to establish a better idea of the accuracy of the speedo in our Captur. Our trust portable SatNav suggested our speedo overreads by 2-3 mph so definitely one of the more accurate examples we have owned in recent years.
Car Speedos are governed by law and regulation by the EU. Satnavs are not. Whilst the satnav probably gives a more accurate representation of speed and distance, especially over faster speeds and longer distance, it is unlikely to serve as a mitigation in a court of law, should one find themselves there for a speeding offence.
So I suggest using the speedo at all times, with due regard to the legal specification laid down my EU and UK law.