Can I charge VAT while I wait for my UK VAT number?

Not on the invoice. Until the 9-digit VAT number arrives by post, you can raise your prices to cover the VAT, then reissue the invoices showing VAT once you have the number. You owe VAT from your effective date of registration, which HMRC confirms in the same letter.

Source: VAT registration: how to register, HM Revenue & Customs, GOV.UK