Hmm, not sure that is correct in the purest anal sense as domestic air travel is subject to VAT but at a rate of zero percent. So according to VAT regulations they are required to issue a VAT invoice if requested, with a VAT of zero. Though in practice the VAT man will not care unless you try to claim VAT back at 15%. The taxman doesn't care about VAT, he just wants to see a receipt to back up any expense deducted by the business.