It looks like they are using session for this purpose. As far as I know there isn't any way to override this.
But you can do some work around for this. Call jQuery ajax function to homepage and redirect on its complete event. While calling the ajax function the cookie (ie session set) will be set on client's browser and after the ajax call redirect user to the page.( The page that won't allow users to visit directly)
<a href="#" onclick="LinkClick()">Click to go</a>
function LinkClick()
{
$.ajax({
type: "get",
url: "http://carfueldata.direct.gov.uk",
dataType: 'jsonp',
complete: function (msg) {
window.location.href=" http://carfueldata.direct.gov.uk/search-new-or-used-cars.aspx?vid=30392";
}
});
return false;
}