You can still send out invoices without your VAT number.  If you go to settings > VAT Settings you can check the box that you are VAT registered and input the VAT number once you have received it.

You can also go to the customers tab and select the customer > Options and check the box to auto include your VAT number on your invoices.  This way once you input your VAT number in the settings section they will begin to appear on any invoices you later send the customer.