//If not they will be going into isolation anyway//
Will they? Why will they? As I have mentioned before, a friend of mine returned home from Europe a couple of weeks ago. He went through the e-Gates at Heathrow (along with everybody else, from everywhere else), was not asked where he'd been or where he was going, whether he was resident or not (and many arriving in the sixty or so passenger flights arriving daily at Heathrow are not). He simply got on the tube and went home. Compare this to the way arrivals are treated in, say Greece (from the FCO website):
"Direct commercial flights between the UK and Greece (in both directions) are suspended from midnight local time on 23 March until 11:59pm Greek time on 15 May. If you are a permanent resident in Greece, and need to return to Greece from the UK, you may be able to travel on any flights organised by the Greek authorities. You will need to provide proof of residency, via a Registration Certificate or Permanent Residence Document. You should contact the Greek Embassy in London for further information.
From 16 March, everyone arriving in Greece from abroad must go into 14 days’ self-isolation. This is mandatory, and the authorities will enforce it by prosecution and fines.
As well as that, some 600 people have arrived in rubber boats since the lockdown began. Most have arrived from living in the most insanitary conditions in northern France. They have been "processed" by the immigration authorities and then "dispersed" (i.e. set free to go where they like) across the country.
//I know it's unpopular but it is the official view until our infection rate is below that of wherever they come from//
We don't know where they've come from. Nobody asks.