We should have closed the borders many months ago but the UK Government did too little too late and played with 'herd immunity' ie doing nothing. Imperial College provided their calculations on 1 March suggesting unacceptable levels of death. Ignored. Represented about 21st March when there was a hasty volte face and a lock down. By the time we had got to that stage the number of infected people in the UK was already at its peak and closing the borders would have only reduced it by a fraction of 1%.....so they didnt. In addition the modelling of how many people would lock down (school attendances, number going out to work, transportation) massively underestimated compliance and with it the number of passengers inbound was less than expected
When the lockdown is eased there will need to be a much smaller pool of infected people and the 'hope' is that track and trace will keep R0 below 1 so it will continue to fall. People will die but it is a a balance between the economy and acts that cause death - not a decision I would want to have to make. At that stage any given influx of passengers has a greater effect. In addition track and trace only works if you can trace so if significant foreigners come in who have no fixed abode or dont speak english or dont use the app the house of cards falls.
It seems the exhortations of a number of people is now being heard and the UK Government will restrict and or control passengers from abroad at that time, bringing them in line with almost all other first world countries. New Zealand is the best model - several industrialists led the politicians, closed down early, total closure of borders (except permanent residents who are quarantined), track and trace so that the country is expected to eliminate SARS-CoV-2 within a month. It will then be able to remove all restrictions but will keep the borders closed until there is a vaccine.