If you have already paid VAT to HMRC but have not received the payment from the customer, you can reclaim it back.
When you write off a transaction the entire (tax+taxable) amount will be debited to Bad Debt.
You can raise a journal entry for the same as follows -
The number “20” is an example for an amount that you wish to reclaim.
This will debit input VAT, and you will be able to reclaim this from HMRC.