Hello Community Users! Just adding some additional thoughts to this thread. As Quickbooks works by calculating VAT as a percentage.
In order to enter an fixed amount for Vat rather than have it calculate as a percentage(if it is a standard Vat transaction) you could use the 100 percent Vat method mentioned in this link
You can adjust in the VAT box on the expense the amount the percentage calculates however that will impact the totals on the transaction.
Alternatively you could adjust the relevant boxes by the amount in the VAT return by making a Vat adjustment if the boxes you need to impact differ to the standard VAT ones. However we'd advise you to speak to an accountant or HMRC to confirm they are happy for you to do that. If you need help with how to make an adjustment just reply here and we can go through that with you.