The click and collect option, you book a day to collect it and then you go there, they scan your QR code, give you the test in a box then you complete it yourself and upload your ID and test result to the website using your phone, then within2 hours you get your certificate.
If you're coming from the US, there are also expresstest options for on site testing at the airports as well as in lots of handy places.
Also, Boots (our version of Walgreens/CVS) offer bookable on site tests for Β£30 and there is a Boots on nearly every major street in the UK!