Covid-19: UK Border Health Measures

I am afraid I simply cannot get my head around the public health mental gymnastics of this policy. If such a barrier is required, why was it not introduced earlier in the outbreak and if it is a contingency measure against a so-called second wave, why apply it to countries with a lower infection rate than we already have? Surely, the answer lies in the Government’s test and trace system, rather than unnecessary economic isolation. I know that my right hon. Friend is not answerable for the public health elements, but can she please tell us, from a Home Office perspective, in the event of air bridges being established: how will it be possible to identify transit and, more importantly, stopover passengers, who may be able to come to the UK through the bridge from higher-risk areas?