Lateral flow tests shouldn’t be used if you have Covid-19 symptoms (high temperature, new, continuous cough, loss of smell/taste) – even if you have had both doses of the vaccine.
If you have symptoms, you must book a Covid-19 PCR test online at https://www.gov.uk/get-coronavirus-test or by calling 119. And remember that you and everyone you live with must self-isolate until test result arrives.