Hi Christineshovlin,
For this, you'd effectively create an invoice with two lines, one being a penny (£0.01) and the 2nd line being (£0.01). Overall price of the transaction comes at £0.00.
On the first line you'd put 20% VAT, on the 2nd line you'd put "Exempt" - This will open-up the VAT DUE box in the bottom right of the screen for your to input the amount of VAT.
The total amount of the invoice will be just the VAT Due.
Thanks,