During observed runs, your observer will expect you to keep strictly to signposted 30 and 40mph limits, especially in built up areas. What matters though is that your speed is approriate for the conditions. In national speed limits you should constantly adapt your speed in accordance with the hazards around you, and exercise your own discretion.
During the test, yes, the assessor will expect you to demonstrate that you know what the speed limit is, and ride up to it accurately if safe to do so. It is worth checking your bike’s speedo using a GPS unit or similar well before the test, as bike speedos can over-read by 10% or more, and you may be able to go faster than you think.