As a sometimes van (small, not a big commercial van) driver the limit is actually 50 on normal roads and 60 on dual carriageways for a start.
50mph is a perfect reasonable abd safe speed on the dual carriageway or motorway and is significantly more fuel efficient. There are at least 2 lanes, you are free to overtake. Also a heavily loaded vehicle needs to be driven more carefully, and if loaded and going uphill cannot physically maintain that speed.
Lorries and tractors etc will often be going slower, especially on long steep hills, and even dual carriageways may have tractors, lorries and even cyclists on, not to mention broken down vehicles, cars towing caravans, horseboxes etc which all need to drive at a reduced speed for safety.
If you can't manage to be prepared for encountering a variety of speeds including a cyclist at 15mph then you are the problem.
Most the 2 lane roads around my area are not appropriate or safe to drive the national speed limit due to bends, potholes, limited range of visibility due to hedges etc. Have you remembered that you are supposed to be able to stop within the distance you can see?
If I was driving under 20 in a 30 it would only because it was a very built up area and eg negotiating parked cars and pedestrians. Not something I'd do generally.
Do you realise that the nation speed limit is a general thing, some roads are speed controlled to lower, the NSL is the default, it doesn't mean that the road has been assessed and deemed safe to travel at 60mph by some wise person, we are responsible for driving safely and according to the road and conditions.
All the single track 'unmarked' lanes are also technically 60 mph limit. Most of them wouldn't even be suitable to drive at 30 on, you need to be prepared to meet another vehicle head on around any corner and need to reverse to a passing space.