Inquisitor - the convention is for runways to be designated by their magnetic bearings because aircraft basically use a magnetic compasses for navigation. VORs for example are calibrated to give magnetic bearings rather than true (except I believe for a few in polar regions where magnetic compasses are useless!). In the UK where magnetic variation is small, the magnetic and true headings are close, but in St Helena the variation is around 17 degrees - the runway headings are taken to the nearest 10 degrees and then the first 2 digits used as designators.
So for the runway on 179 degrees true, the magnetic heading would be about 196 degrees, taking the nearest 10 degrees makes it 200. Hence the designator "20".
That aerodrome diagram is interesting in that it gives the threshold elevations in metres (above sea level). Usually aircraft are given this figure in feet as that is the unit used normally for altitude or height information (except in places such as Russia where metres are used). So runway 20 has a threshold elevation of 1014 feet and 02 an elevation of 997 feet.
Chance of a bit of confusion there "QNH 1025, threshold elevation 1014 feet" - the ATCOs will have to be sure aircraft get it right! Get the QNH and elevation swapped around and you could have an error of 330 feet on approach.