An enforced 14 day quarantine will surely see PAX coming to the UK grind to a complete halt other than those still trying to get home. Who is going to book a flight knowing what is to come on arrival.
Having both open borders without any screening and a lockdown in place is a bit like having two sprung leaks in a sinking ship. One leak gets plugged and the other is left. I really hope the authorities have been making good use of the last few weeks with a view to implementing the same kind of airport screening that has proved successful in certain other countries.