We always email customers a copy of their VAT invoice when each order has been completed.
If you need an invoice copy for an older order then you can get one from your account dashboard. Navigate to the My Account area (you might need to log in) and select orders from the options on the left. You will be presented with a list of all your historical orders. Click the view button of the order for which you need the invoice and you will be shown the order details. At the bottom of this page you have the option to print an invoice.