So to record this you need to do a 100% vat here are the steps to do this.
You can easily enter a VAT only transaction. To do this, click on the + sign and select the transaction type that you need.
One the first transaction, choose the account you want to use. For the rate, enter £1, and for the VAT code, select the 20% Standard code.
On the second line, select the same account. For the rate, enter -£1, and choose an Exempt code.
The total of the invoice will be 0. You can then edit the amount in the VAT box to whatever you need it to be.