Please make yourself aware of the HMRC rules relating to how soon you can write off a bad debt.
Write off a bad debt by producing an internal credit note for the amount being written off. See Credit a Customer Account.
The V.A.T. analysis must be the same as the invoice(s) being written off – i.e. use the same V.A.T. codes (and rates) and net amounts. Use the Daybooks > Sales> Credit a Produced Invoice facility to be sure you are reversing the VAT correctly. You can change the nominal code to bad debt write off or provision account before posting.
Consider whether the amount is a bad debt or uncollectable for some other reason – i.e. the goods never arrived – in which case no sale was made – in this sort of case you can immediately credit the sales account and VAT and send the credit to the customer.
Otherwise, at the appropriate time, charge the net amount to bad debt expense account and debit the VAT account.
Allocate the credit note against the invoice(s).