Hi @cedev
Thank you for your suggestion.
At present, it’s not possible to have the client VAT number on the invoice unless you’re reverse charging the invoice (see this thread for details).
We can certainly consider this if there’s enough interest, although it’s worth noting that it’s not a requirement by HMRC to show your client’s VAT number on the invoice.
In the mean time, you may wish to add this to the notes box on the invoice.