Those flying into UK are generally Brits being brought home.
When to test is an issue but is complicated. Temperature checks are not too reliable and don't identify non/pre-symptomatic cases who may be carrying the virus. Test results may take 2-3 days to come back so we'd need to quarantine them all until the results are back or at least require them to leave contact details.
Tests should be done before they fly- maybe a few days before - since a passenger X who gets on with the virus could infect others on the plane but those who get infected by X won't show up in tests on landing as it can take a few days to show.
In general these people are coming from countries where the incidence of infection is lower than ours, so they are not high risk.
As long as these Brits follow social distancing when they return like the rest of us they are no different to us.
These are the arguments the scientists have used.
The belt and braces approach is to quarantine all of them on landing for 2 weeks in a hotel Single room, food left at the door)
We could test on