Code snippet to validate UK VAT IDs with Barns2uks EDD EU VAT plugin. Requires https://vatsense.com API KEY

uk-vat-id-validation.php

<?php
//License is public domain, use to your hearts content. I take no ownership or responsibility
use Barn2\Plugin\EDD_VAT\VAT_Check_Result;

add_filter( 'edd_vat_number_check', function ($result, $vat_number, $country_code ) {
    if($result->error === VAT_Check_Result::INVALID_INPUT && $country_code==='GB') {
        if(strlen($vat_number)!==9 || !filter_var($vat_number, FILTER_VALIDATE_INT)) {
            return $result;
        }

        /** @noinspection PhpUndefinedConstantInspection */
        $basicauth = 'Basic ' . base64_encode( 'user:'.VATSENSE_API_SECRET );

        $headers = array(
            'Authorization' => $basicauth,
            'Content-type' => 'application/json',
        );
        $request_data = ['headers' => $headers];

        $response = wp_remote_get('https://api.vatsense.com/1.0/validate?vat_number='.$country_code.$vat_number, $request_data);
        $body = wp_remote_retrieve_body($response);
        $code = wp_remote_retrieve_response_code($response);
        if(200 === $code) {
            try {
                $data = json_decode($body, true);
                $result->valid = filter_var( $data['data']['valid'], FILTER_VALIDATE_BOOLEAN );
                if($result->valid) {
                    $result->name = $data['data']['company']['company_name'];
                    $address = explode( "\n", $data['data']['company']['company_address'] );
                    $result->valid = filter_var( $data['data']['valid'], FILTER_VALIDATE_BOOLEAN );
                    $result->address = implode( apply_filters( 'edd_vat_check_result_address_separator', ', ' ), array_filter( $address ) );

                    $result->error=null;
                } else {
                    $result->error = VAT_Check_Result::INVALID_VAT_NUMBER;
                }
            } catch (Exception $e) {
                $result->error = VAT_Check_Result::API_ERROR;
            }

        } elseif(400 === $code){
            $result->error = VAT_Check_Result::INVALID_INPUT;
        }else {
            $result->error = VAT_Check_Result::API_ERROR;
        }
    }
    return $result;
},0,3);