https://www.bbc.com/news/uk-55582116
Airlines are now used to screen pax on departures based on predeparture requirements. My guess is that anyone without a test will be denied boarding rather than allowed to board with a 500 fine.
What is interesting is that this also applies to boat and car arrivals. It would suggest that anyone would be allowed to enter by just paying the fine. It could also be that boat and tunnel operators might deny access without the test. Will be a big mess at first.