If you go with the english government's recommended sites DO NOT choose Circular1 Health...... I didn't get my results back for 10 days and then they were inconclusive.
Better 2 Know have got good trustpilot ratings. I'm getting their antigen tests for return to EU (France). I don't think you can do a drop in test because you need to register an address, but I could be wrong about that. If you arrive , do the test and send it off the same day, You should get results back within a day or two anyway.