This way
string[,] GeoLocation = new string[,]{
{"-0.174464","51.542180"},
{"-0.172925","51.548507"},
{"-0.178464","51.556919"},
{"-0.178487","51.556902"}
};
int index = new Random().Next(0, GeoLocation.Length - 1);
string lat = GeoLocation[index, 0];
string lng = GeoLocation[index, 1];