There was a clear U-turn in Government policy when the Government realized their initial approach was far too lax and they'd messed up badly, partly due to a slightly arrogant 'UK knows best, not these Johnny Foreigners' approach, no matter what evidence from elsewhere in the world might suggest. Since that point though, I think the UK Government policy has been pretty consistently strict (at least publicly, where certain Government advisers have not followed that policy is another topic....) and mostly the right choices. I'm not sure I see 'swerving from side-to-side' here, just the UK government doing a much better job of actually reacting to events. If you drive in a straight line forever without turning, you are liable to eventually crash, in your metaphor.

Testing everyone doesn't help you if someone gets infected by someone else on the flight and the case is not yet visible, potentially leading to a number of infected passengers from this flight entering the country and spreading it further.