A soft credit check is when a lender checks the credit file or report that a credit reference agency (CRA) holds on you. This is in order to offer pre-approval (not full approval) for a loan, or in this case a mortgage. Lenders do this to get an indication of your credit health. It doesn't impact your credit score. And the only person who can see a soft credit check on your file, is you. These credit reference agencies are companies like TransUnion, Experian, or Equifax.
A soft credit check is necessary for a Mortgage in Principle. This is because the lender needs to check the information on your credit report. And decide whether offering a loan could be appropriate for your circumstances and affordability. Find out more about how to improve your financial health.