I don't think anyone can put a number of deaths on what would happen if you flood every hospital in the UK, I would have thought 500,000 would be a bit conservative tbh.
I'm actually against this, travelers in and out of the UK should have been put in the airport hotels fed 3 times a day and then let out with a negative test after 10 days. As far as I know a single test is pretty useless because it can't tell if the virus is in incubation and if you have a negative test surely you will be a bit less careful. If you have to go abroad then the price for that is 10 days in a hotel it might even put some chancers off going.