As previously mentioned, your car's speedo is not calibrated whereas state/police operated speed measuring devices have a legal requirement for frequent periodic calibration, therefore its accuracy can be readily checked.
The usual rule of thumb for triggering a camera is;
Speed Limit + 10% + 2 (in this case 30 + 3 + 2 = 35)
but any speed over the posted limit is an absolute offence. Therefore you could be prosecuted for travelling at 31 mph but this is far from usual. Some speed cameras have been reported as being set to 33 mph.
Your speedo appears to under-read your road speed which is actually unacceptable in law (whereas an over-read, with a limit of +10%+2.5mph, is acceptable). This means that when driving at a true 35mph the speedo is allowed to indicate a speed in the range 35 to 41 mph and should never indicate a speed below the true speed.
The apparent under-read is based on your SatNav, which is far more accurate than the speedo, reading (much!) higher and the laws of physics dictating that a noisy speedo cable is very likely indeed to affect (lower) the readout.