In less than one day, a couple of the test providers have already been taken off the list and one added.
If you eventually find someone who can provide the test, (requires numerous calls and e-mails until you get someone with capacity), you need to arrange a courier pick-up and return of the test to the laboratory on day five of isolation, in order to get a result by day six - saving four days of quarantine. That might cost overall anything up to £500 (two-way couriers/taxi and the test itself).
Forget the post, complete waste of time and money, you might eventually get released by day nine in practice.
Alternatively, be on 'an overseas business trip that is absolutely essential and proven to win a £250,000,000 order for 'UK PLC' or save the jobs of 50+ UK-based employees' and that's the only other way to get out of quarantine (unless on the exempt list of certain critical professions).
The airline industry is certainly not saved on this basis.