Your use of speed on a driving test will demonstrate how you are acting and reacting to the other road users. This will be evidence of how you are reading the road. But also how you are reacting to what you see.
An examiner can only mark what he sees at that time and place. To adopt a plan where you are extra cautious by going particularly slowly just to be extra safe is wrong. The flip side is if you are driving to fast for the conditions. If you are driving in a particular way just for your test this is wrong. A natural smooth and flowing drive is what should be aimed for.
When out on test if the examiner is using sat nav this may help you if you are not sure of the speed limit. Check to see what the sat nav says. That said beware of temporary limits such as with roadworks. Also watch for advisory limits. A limit posted outside the car will always override a limit given by a sat nav.
At junctions if it’s safe to go, you should go. The examiner needs to know if he got stuck behind you at some point in the future he will not be waiting all day. You are not expected to be an expert but you are expected to be safe.