Hi PeterS
Thanks for interesting in our plugins.
As you said, you can't validate your UK VAT through VIES. We're aware that people can't enter the company info during the checkout so, we added creating a workaround to our TO-DO list.
Meanwhile, you can buy the plugin without the company info. After that, enter your profile and change the company info (see the image)
Now, you'd be able to download the invoice with your company's info.
Hope this helps,
José Anrés