On raising a tax invoice (vat)

Not necessary, you can raise invoice before delivery of goods.

so much of companies raise the after delivaring the goods but that is not necessary

Sir, thank for your reply, and i got it later that we can raise the invoice after the deliver but within the 30 days from the date of delivery 

https://www.nibusinessinfo.co.uk/content/time-limits-issuing-vat-invoices