Code and KML for partial UK polygon.

coordinate cutdownpoly[17] = 
{ // Latitude,Longtitude
 53.166F,-2.166F,  // 0 15K N Stoke-On-Trent (datum)
 53.166F,-0.1F,   // 1 30K W Skegness (125K east of datum)
 52.833F,-0.1F,   // 2 5Km N Spalding
 52.7F,0.333F,   // 3 5Km SW Kings Lynn
 52.87F,0.7F,   // 4
 52.833F,1.25F,   // 5
 52.666F,1.5F,   // 6
 52.416F,1.5F,   // 7
 52.083F,1.166F,   // 8 Westerfield N of Ipswitch
 51.666F,0.3F,   // 9 5Km N of Brentwood
 51.666F,-0.666F,  // 10 10K NW High Wycombe
 51.166F,-0.666F,  // 11 Godalming
 51.166F,-2.166F,  // 12 Warminster
 52.333F,-2.166F,  // 13
 52.333F,-1.666F,  // 14
 52.666F,-1.666F,  // 15
 52.666F,-2.166F,  // 16
};

<kml xmlns="http://earth.google.com/kml/2.0">
<Placemark>
  <name>Small UK</name>
  <Style>
    <LineStyle>
      <color>ff0000ff</color>
    </LineStyle>
  </Style>
  <LineString>
<altitudeMode>absolute</altitudeMode>
    <coordinates>
 -2.166,53.166
 -0.1,53.166
 -0.1,52.833
 0.333,52.7
 0.7,52.87
 1.25,52.833
 1.5,52.666
 1.5,52.416
 1.166,52.083
 0.3,51.666
 -0.666,51.666
 -0.666,51.166
 -2.166,51.166
 -2.166,52.333
 -1.666,52.333
 -1.666,52.666
 -2.166,52.666
    </coordinates>
  </LineString>
</Placemark>
</kml>

C

A C function to test if a point (x, y) is inside a polygon (poly, points). Returns 0 if outside, !0 if inside the polygon. poly points to an array of x, y coordinates of the polygon. points is the number of points in the array.

int pointinpoly(float *poly, int points, float x, float y)
{
        float *p = poly;
        float *l = &poly[points * 2 - 2];
        int c = 0;
 
        for(; points; points--, l = p, p += 2)
        {
                if(y < p[1] && y < l[1]) continue;
                if(y >= p[1] && y >= l[1]) continue;
                if(x < p[0] + (l[0] - p[0]) * (y - p[1]) / (l[1] - p[1])) continue;
 
                c = !c;
        }
 
        return(c);
}