Before my VAT registration date I submitted various quotes, won the jobs and almost completed all of the work. (They are non VAT registered people that I have done the work for.) Now I have my vat number and am VAT registered it's time to invoice those clients. They will be very annoyed if I now add VAT to their bill because it was never mentioned at the time. (Some of these jobs were started weeks & months before my VAT registration date and I'm based in the UK).
Quoting from https://www.gov.uk/vat-registration :
While you wait
You cannot charge or show VAT on your invoices until you get your VAT number. However, you’ll still have to pay the VAT to HMRC for this period.
You should increase your prices to allow for this and tell your customers why. Once you’ve got your VAT number you can then reissue the invoices showing the VAT.
Of course, it's too late to follow that advice. I imagine that if you add VAT to your invoices now, this would constitute a breach of contract (i.e., you told your customers one price but are now invoicing them for a higher price).