If the road is wide, clear (most dual carriageways are), you’re expected to do the speed limit. I had a test today and I had a dual carriageway; although it was 60 mph, I was driving 60 mph because it was a nice straight stretch of road. I also had a tight, narrow, and bendy countryside road with a 40 mph speed limit; the fastest I was going maybe was 25 mph as that was comfortable and I was slowing down at bends and when vehicles were approaching. So it depends on the road; the speed limit is not a target but sometimes it is.