The views expressed in this post are my own personal (HRA protected) views, and are not representative of any organisation I have any involvement with.
As per the HMRC guidance, until you receive your VAT registration number you must not charge VAT, or show VAT as a separate item on your invoices. To make sure that you do not lose income in the period after you applied for VAT registration but before you receive your VAT registration number, you should increase your prices by the VAT rate relevant for your goods or services, and explain to your customers why you are doing so.
Once you are registered, you can then reissue those invoices, amended to show your VAT registration number and the VAT charged. This will ensure that your VAT-registered customers can reclaim the VAT that they have paid.