Take a look at how I did my test from a hotel after arriving in America - this is permitted within the Cunard rules:
My experience from the recent 1st May eastbound TA may help you. Obviously requirements for Cunard are subject to change at short notice.
I booked a Fit To Fly Antigen Test With Observation from Medicspot for £19.99. With a coupon code I found by searching the internet this came down to £16.50.
Cunard were happy with the test being performed any time in the previous 2 days, so for a Sunday departure I was ok to do it anytime on Friday. If I recall, test appointments were available any time between 9am and 6pm UK time.
Test was done over zoom, both myself and my travelling companion were able to do the test on the same zoom session - I did this on my iPad with no problem. They told me the certificate could take up to 2 hours, but it arrived within 5 minutes.