VATREC6072 | Transmission of invoices by fax: Customers
From HM Revenue & Customs · VAT Traders’ Records Manual
The customer may use the faxed invoice as evidence for input tax deduction. But you should consider verifying orders, payment, etc, on selected invoices received by fax as a safeguard against double claiming against both original and fax copies. You should also bear in mind that plain paper fax machines or PCs with a fax facility produce very high quality documents, which may not be easily recognisable as faxes.