I just tried to order some Covid lateral flow tests on the UK Government website but Avast blocked my connection as it detected “Phishing” by the website. I was not given the option to override the block.
Reporting Possible False Positive File or Website - https://www.avast.com/false-positive-file-form.php.
Since you don’t give the url being blocked it isn’t possible to investigate here. Attach a screenshot of the Avast Alert Window, with the more options element open. See attached image on how to do this.