Or it's a quick invoice that doesn't fully comply with the requirements for being a proper VAT invoice, which require...
"an invoice number which is unique and follows on from the number of the previous invoice - if you spoil or cancel a serially numbered invoice, you must keep it to show to a VAT officer at your next VAT inspection
the seller's name or trading name, and address
the seller's VAT registration number
the invoice date
the time of supply (also known as tax point) if this is different from the invoice date - see below
the customer's name or trading name, and address
a description sufficient to identify the goods or services supplied to the customer
For each different type of item listed on the invoice, you must show:
the unit price or rate, excluding VAT
the quantity of goods or the extent of the services
the rate of VAT that applies to what's being sold
the total amount payable, excluding VAT
the rate of any cash discount
the total amount of VAT charged
If you issue a VAT invoice that includes zero-rated or exempt goods or services, you must:
show clearly that there is no VAT payable on those goods or services
show the total of those values separately"
http://www.hmrc.gov.u...ging/vat-invoices.htm