I took my fit to fly antigen test yesterday at Boots Piccadilly. Very easy. (This is for my return to the U.S )
Book online. Make an appointment online. Show up. That's it. (Bring your passport and test registration number.) YOU MUST HAVE YOUR PASSPORT. No other ID is accepted.
I arrived a little early and there was a line. Because I had an appointment I was told to wait. Many people arrived without appointments and were told to come back later. My appointment was for 11:30 but it became first come, first serve for those with appointments. I was seen at 12:00.
When it's your turn, they ask if they can authorise payment (30 GBP) and they test. (You provide payment information when you register online but it's not taken until you test.) It took about 5 minutes.
Results sent via email within 30 minutes. A suggestion...hang out near your test center. After 40 minutes, I didn't get my results. I went back and they resent it. You need the results (certificate) to get on the plane.