When you apply you agree to this legal declaration:
https://www.ucas.com/ucas-undergraduate-declaration-2019-entry"If we, or a university or college, have any reason to believe that you or your referee have:
"- left out any relevant information, including qualifications you have completed, qualifications with an unsuccessful grade or qualifications for which you are still awaiting results
"- given false or misleading information
"We may take any necessary steps to check with you and other parties, including universities, colleges and examination and awarding bodies whether the information you have provided is accurate or complete.
"We have the right to cancel your application without refunding your application fee if we determine (having carried out any necessary checks), or have reasonable belief, that your application contains false information.
"If you have any reason to believe that information we hold about you is not true, complete and accurate, you must tell us."
You can't withhold information and then declare that you've included everything asked for - that's fraud. It will get found out and lying is considered much more of a problem than a low grade.