I confess to be a bit ITK on this. The issue is two-fold.
Firstly the Dept of Transport has moved the governance of this to the individual transport providers (Airlines, Shipping, Trains) instead of the UK Border Force. There are several reasons for this, mainly the Border Force is severely undermanned and would not be able to cope. Now, imagine all the airlines for example that fly to the UK. They each need to have something in place (and they do not know what yet) at the point of departure which btw could be anywhere in the world that does not have a flight ban to the UK. On top of this, they will be fined £4k for each passenger that arrives without the required test
Secondly, the DofT has not, to date been precisely clear on what evidence of what test is required. For example, does it have to be in English? Is there and age limit (ie Under 5's) and what type of test is actually required. At the moment this is not clear.
It's a mess and I'll resist making the point that they have had 9 plus months to think about it