mypmpro-dont-vlidate-uk-tax.php

<?php
/**
 * This ensures that the UK can be selected from the dropdown to add Tax to the
 * membership but will not validate the VAT number.
 * 
 * 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 mypmpro_dont_validate_uk_tax( $valid ){

	if( ! empty( $_REQUEST['country'] ) && $_REQUEST['country'] === 'UK' ){
		return true;
	}

	return $valid;

}
add_filter( 'pmprovat_skip_validation', 'mypmpro_dont_validate_uk_tax', 10, 2 );