Edge markings are reccomended on unlit roads with kerbs.
On recently build rural Single Carriageway roads built to current construction standards the edge of carriagway lines will be about 1m from the actual edge of the carriageway. The purpose of the gap (known as a "hard strip") is to provide for drainage of the road it also helps to keep the white line (and any cats eyes) away from the edge of the road where debris often builds up and obscures the markings. Cyclists will often take advantage of them to keep out of the way of traffic - if you were able to stop and examine the road surface you would find that this area has a slightly different construction from the rest of the road - generally it is much shallower and weaker in construction as it does not usually have to bear the weight of vehicular traffic.
As well as being broken at entrances there may also be short gaps to allow water to drain through to nearby surface drains.
Further information is available in
Chapter 5 of the Traffic Signs manualIt is legal to cross the line unless unless you are on a special road (mostly motorways but also some standard dual carriageways were pedestrians and horses are not permitted such as the A720), where they usually mark the edge of a hard shoulder.
If there is a proper (no vehicles allowed) cycle lane the wider solid white line is accompanies by the road sign illustrated in the highway code.