If you just need to control the country, you only need a form with a country select list on it. store the selected country in a session variable, then use an if statement to redirect your site pages to another page if the country session is not one of the allowed countries, for example:
<?php
@session_start();
$allowedCountriesArr = array();
$allowedCountriesArr[] = "US";
$allowedCountriesArr[] = "UK";
if(!isset($_SESSION['alowedCountries']) || !in_array($_SESSION['alowedCountries'],$allowedCountriesArr)) {
header("Location: index.php");
}
?>