Add phone and shipping method fields to the checkout page, and change order of countries in Country field, with UK as the default.

pmpro-customizations.php

<?php

/**
 * This recipe adds custom phone and shipping method fields to the checkout page.
 * It will also change the order of countries in the Country field and set the UK
 * as the default.
 * 
 * It is intended for use with the Register Helper and Shipping Address Add Ons:
 * https://www.paidmembershipspro.com/add-ons/pmpro-register-helper-add-checkout-and-profile-fields/
 * https://www.paidmembershipspro.com/add-ons/shipping-address-membership-checkout/
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmprorh_init() {
	global $pmpro_countries, $pmpro_default_country;

	//don’t break if Register Helper is not loaded
	if ( ! function_exists( "pmprorh_add_registration_field" ) ) {
		return false;
	}

	// Set the UK as the default country
	$pmpro_default_country = 'GB';

	$priority_countries = array(
		'GB',
		'US',
		'NL',
		'FR',
		'DE',
		'ES'
	);

	$countries_priority  = array(
		'GB' => $pmpro_countries['GB'],
		'US' => $pmpro_countries['US'],
		'NL' => $pmpro_countries['NL'],
		'FR' => $pmpro_countries['FR'],
		'DE' => $pmpro_countries['DE'],
		'ES' => $pmpro_countries['ES'],
	);

	$countries_rest = array();
	foreach( $pmpro_countries as $key => $value  ) {
		if ( ! in_array( $key, $priority_countries ) ) {
			$countries_rest[$key] = $value;
		}
	}

	$pmpro_countries = array_merge($countries_priority, $countries_rest);

	$phoneField = new PMProRH_Field(
		'phone',
		'text',
		array(
			'label'    => 'Phone',
			'required' => true, // make field required, set value to false to make it optional
			'profile'  => true, // show in user profile,
		)
	);
	pmprorh_add_registration_field(
    'after_email', // location on checkout page
    $phoneField // PMProRH_Field object
	);

	$shippingMethodField = new PMProRH_Field(
		'shipping_method',
		'select',
		array(
			'label'    => 'Shipping Method',
			'required' => true, // make field required, set value to false to make it optional
			'profile'  => false, // show in user profile,
			'options' => array(
				'launch_offer_within_uk' => '*Launch Offer Shipping Cost – £0',
				'launch_offer_international' => '*International Launch Offer Shipping Cost – £9.99',
				'standard_shipping_uk' => 'Standard Shipping within UK (3-5 days) – £4.80',
				'express_shipping_uk' => 'Express Shipping within UK (1-2 days) – £8.54',
				'international_shipping' => 'International Shipping (7 – 14days) – £21.92',
			)
		)
  );
  pmprorh_add_registration_field(
    'before_submit_button', // location on checkout page
    $shippingMethodField // PMProRH_Field object
  );
}
add_action( "init", "my_pmprorh_init" );