It is right to say above Drive according to the conditions. There is no need to do twenty along a long clear wide piece of road that you can clearly see no-one is on. This is London wide, by the way, but there has to be a test case to be brought by someone brave and rich enough to test the legality. We need a rich footballer's legal team to test this one. They can get their clients off anything, and indeed taught us for example that if a double yellow line is not 'ended' properly with two lines perpendicular to the road, it ain't a proper double yellow line. I would always cite the Institute for Advanced Driving/Motorcycling because you can not be expected to ride according to more than one set of rules. If one person rides at 20 while everyone else is doing an otherwise safe 33mph, you are the one causing the danger!