The Isle of Sheppey lies in the Thames estuary (outside the flood barrier) and is low-lying. It is a place of vulnerability, threatened by high tides and shifting coast. The image is of some wooden 'shack' properties (actually holiday lets) situated right on the edge.
The edge is representative of several aspects of this situation - the beach/land divide marked out by a fence - the beach/sea relationship punctuated by the wooden groynes that seek to maintain some stability - the horizon marking out the sea/sky boundary along with the hint of what it is that lies beyond - the dawn light which marks the transition between night/day offering a future hope, while the 'red sky in the morning' gives a warning of possible threat to come.
For Sheppey residents the sharpest edge that they live on is likely to do with climate change with rising sea levels, but the new day always offers hope. The red sky may offer a warning, but this can be a spur to action.
For those of Christian faith the brightening horizon and all that is hopeful beyond it are a reminder of what it is that God promises. The sky may be seen as a call to a change of heart, so that God's promises, seen in the life, death and resurrection of Jesus, might be real for us within the present time. The sharpest edge here is that between this world and life and the next, but this is not an edge to fear because God is on both sides!