They're not. If in the opinion of the examiner it is perfectly safe to drive at the posted limit you will be penalised for not doing so. Even though it is not a legal requirement to drive to the limit the purpose of the test is to show the examiner that you are capable of doing so if conditions allow.
Yes, they are. If the examiner is an OLAM (and probability is that many will be), that matter of opinion will result in people failing for "not making progress" or whatever the exact current cipher is for failing anyone who doesn't treat a speed limit sign as a target speed.
I was given a "minor" fault mark for not passing the school at
https://goo.gl/maps/3bKTWKrwYQS2 at the full 30mph - and I see that's now a 20 limit!
- and here's where I failed for not making progress:
https://goo.gl/maps/h69wMvwLpXr - I had pulled away from the lights at the crossroads with the ring road at the top of the slope and did not accelerate to the full 40mph because it was blatantly obvious from the speed, positioning and general behaviour of the vehicles ahead that several were about to turn left into Whiffler Road, some without indicating before the last minute. Instead of being on their bumpers and braking, I arrived at the junction just as it cleared and I continued straight on and passed the 30mph limit signs doing 30mph. At that point, I think I'd lived in the city for three years and had cycled around most of it. I knew that road fairly well (it runs between a couple of retail parks and a district shopping centre) and learnt on a bike that if I didn't ease off and watch out, I'd be braking to avoid cars turning in. Vehicles crash at that junction fairly often - 13 reported collisions in the last decade, and I'd bet on a fair number of unreported ones too.
When I passed, my "minor" for speeding was a bit further along on the same road! I misjudged accelerating to 30, went a little over and tried to decelerate without the examiner noticing - didn't get away with that, then!
I'm OK with the idea of treating limits as pseudo-targets on simple roads out in open country and on motorways, but I feel more caution and respect for non-motorised users should be encouraged when driving in urban areas and the practice of the current driving test seems a problem.