The encoded polyline in the map below approximates the coast of Britain with about 50,000 data points. The data was extracted from a shapefile using LandSerf and then saved in text format for use in my polyline encoding form. Virtually all of the points are encoded in the path. In fact, the data is not very precise, as you will see if you zoom in past about level 12.
If you want the data, in comma-separated text format, it's right here and about 1M.