When your turnover level reaches the VAT threshold or you apply for voluntary VAT registration
Application for registration for VAT is taking at least 3 weeks from the date of application to provide a VAT registration number.
Ensure your invoicing procedure takes the date of supply into account when you have a gap between date of registration and when your VAT number arrives.
Issue a bill for the goods including an amount equal to VAT but don’t call it VAT. The extra line on your invoice should read “This charge is in lieu of VAT pending registration. You cannot reclaim it on your VAT return until we send you a VAT invoice”
When you have your VAT registration number send a revised invoice showing the extra amount as VAT.