The reason you get a conic shape is because of the 'worldmap' syntax below which sets the latitude and longitude limits.
worldmap([55 90],[-10 40]);
By playing around with the latitude and longitude limits you would be able to get a rectangular outline of the map. For instance -
worldmap([-50 90],[160 40]);
load coastlines
plotm(coastlat,coastlon)
geoshow(coastlat,coastlon)
will result in the image -
