believe me, you are allowed. Randox certificate is fully accepted. It does not say self administered on the certificate.
I've entered Thailand twice with a self administered test. Don't forget you were tested on arrival too, until about 1 month ago. So you'd be stupid to try and fake the pre flight test. It does say RT.PCR on the cert.
I've also used Randox home tests for during UK self isolation periods, when entering UK. Day 2, Day 5 & Day 8.
if in England, Randox do a self test home pre flight PCR kit. Posted to your home and you drop swab into one of their drop boxes. Rapid results. Certificate emailed to you. You would need to check if they have a drop box near you or your work.