These days businesses rely upon various other companies to help administer sales.
This might be companies such as Just Eat Hungry Horse or Deliveroo to facilitate orders or various card companies to facilitate payment.
Any one of these might be approached by HMRC to provide them with details of your throughput to enable them to estimate your total receipts.
They can then compare this data with what you have told them either in your tax return or in your VAT return.
Make sure all your sales are properly recorded so that all your data and returns are consistent so that HMRC cannot catch you out.