According to Jonathan Sumpton, Ex Supreme Court Judge, the Act itself is likely not legal. The Coronavirus Act 2020 and the draconian measures therein were legislated under the Public Health and Control Of Disease Act 1984. Sumpton, suggests that the correct Act to have legislated under would be The Civil Contingencies Act 2004 but that Johnson and his communist chums opted for the Public Health Control of Disease to AVOID parliamentary scrutiny. In other words, they wanted to remove the possibility of a revolt by MPs. The Civil Contingencies Act 2004 has a built in requirement of 30 day reviews (voted on by MPs) and can be amended or revoked by Parliament at any time. So given that dictatorships are difficult to form were democracy exists, it becomes obvious why they opted for the Public Health Act. You can listen to Sumpton discussing this here https://www.youtube.com/watch?v=I5ibHtut9zA