“6(1) The provisions of this section shall apply … for determining the time when a supply of goods or services is treated as taking place for the purposes of the charge to VAT. … (3) Subject to subsections (4) to (14) below, a supply of services shall be treated as taking place at the time when the services are performed. (4) If, before the time applicable under subsections … (3) above, the person making the supply issues a VAT invoice in respect of it or if, before the time applicable under subsection … (3) above, he receives a payment in respect of it, the supply shall, to the extent covered by the invoice or payment, be treated as taking place at the time the invoice is issued or the payment is received.”
“1(1) Subject to sub-paragraphs (3) to (7) below, a person who makes taxable supplies but is not registered under this Act becomes liable to be registered under this Schedule – (a) at the end of any month, if the value of his taxable supplies in the period of one year then ending has exceeded£67,000 ; or …”
“Where a payment is made on account before the goods or services are supplied, VAT shall be chargeable on receipt of the payment and on the amount received.”