If you receive a negative test result this releases you from quarantine as per UK Govt guidelines. 

If you test positive you will be required to quarantine for a period of time which is currently 10 days from your positive test result.  

Please check https://www.gov.uk/government/publications/coronavirus-covid-19-how-to-self-isolate-when-you-travel-to-the-uk/coronavirus-covid-19-how-to-self-isolate-when-you-travel-to-the-uk for up to date Govt guidelines.