woocommerce-redirect-to-checkout.php

<?php
/**
 * Plugin Name: WooCommerce Subscriptions Redirect to Checkout
 * Description: Redirect customers to the checkout page when adding a subscription to their cart (rather than the checkout page, which is the default).
 * Author: Simon Kelly
 * Author URI: http://www.renegade-empire.com
 * Version: 1.0
 * License: GPL v2
 */

function re_redirect_to_cart_page( $url ) {

	// If product is of the subscription type
	if ( is_numeric( $_REQUEST['add-to-cart'] ) && WC_Subscriptions_Product::is_subscription( (int) $_REQUEST['add-to-cart'] ) ) {
		// Redirect to checkout instead
		$url = WC()->cart->get_checkout_url();
	}
	return $url;
}
add_filter( 'woocommerce_add_to_cart_redirect', 're_redirect_to_cart_page', 20 );