Friends of ours had Rapid Antigen Results accepted that were not tests on the approved list.

I don't know what it is like for testing turnaround time in the UK, but we got in trouble getting a PCR Test at a Walgreens on a Friday in the States for travel on Tuesday. We didn't get results until midday Tuesday which created some last minute headaches on Monday evening and with the portal...

Just a heads up that getting PCR tested on Friday/Saturday/Sunday in the States does sometimes seem to lead to delays in results