After VAT registration, when should I start charging VAT on goods and services?

You must start charging VAT on anything you sell from the day you register for VAT with HMRC, not the day you receive your certificate. You should raise invoices for all sales made after VAT registration, and upon receipt of your certificate, you can add your VAT number to the invoices, separate the sale and VAT amount and re-issue the updated invoices to your customers.