Redirect homepage depending on country code.

functions.php

<?php // Do not include this if already open!

/**
 * Return the country code.
 *
 * @since 1.0.0
 */
function get_user_country_code() {
	return 'GB'; // This is main United Kingdom front page.
}

/**
 * Check either landing pages is in view.
 *
 * All landing page should exists with country code slug.
 *
 * @since 1.0.0
 *
 * @return bool Is landing page exists?
 */
if ( ! function_exists( 'is_landing_page_exists' ) ) {
	function is_landing_page_exists() {
		return is_page( array_map( 'strtolower', ['FR', 'BE', 'DE', 'IE', 'NL', 'CH', 'SE'] ) );
	}
}

/**
 * Redirect homepage depending on country code.
 *
 * @since 1.0.0
 */
function mytheme_change_homepage_by_country_code() {
	global $wp;

	if ( ( ! is_admin() || defined( 'DOING_AJAX' ) ) && ! defined( 'DOING_CRON' ) ) {
		$country_code = get_user_country_code();

		if ( ( 'GB' === $country_code && ! is_front_page() ) && is_landing_page_exists() ) {
			wp_safe_redirect( home_url() );
			exit;
		} elseif ( ( 'GB' !== $country_code && is_front_page() ) || ( is_landing_page_exists() && $wp->request !== strtolower( $country_code ) ) ) {
			wp_safe_redirect( home_url( sprintf( '/%s/', strtolower( $country_code ) ) ) );
			exit;
		}
	}
}
add_action( 'template_redirect', 'mytheme_change_homepage_by_country_code' );