After much searching and sifting out expensive options, I found a very fast and inexpensive pre-flight Covid test you can get in 50 different UK (England, Scotland) cities. I took the test in Bath as I was departing for my flight back to San Francisco the next morning, but it looks like this service also has testing clinics throughout the UK.
I'm just posting here to save people time and expense since our Bath B&B owner (Peter at Marlborough House) didn't have the latest details and there are lots of predatory (expensive) Covid testing if you aren't in-the-know. I am not affiliated with the service, but from my personal experience, I highly recommend it.
I went to officialrapidtests.com and clicked the "In Clinic Testing (PCR & Antigen)" button ("Find Test Centre").
N.B. To fly back to the States, all you need is an Antigen test. PCR tests are also accepted, but they take lab work -- the Antigen test is like your home tests, but it's proctored by a doctor, and MUCH faster.
From there, select your city and they'll offer you a local option. In Bath I went to the DAM Clinic at 3 Milsom Street. They do testing 7 days a week from 8AM to 4PM with appointments booked every 15 minutes, and you get your Antigen test results back by email within 2 hours (mine took 1 hour). Price as of June 2022 was £22.49 -- much cheaper than nearly all other options I found, including testing at Heathrow.
Select the "In-Clinic Lateral Flow Antigen Fit to Fly Test (Results within 2 hours)" option, fill in your info, book the timeslot, and be sure to show up on time. While they do take walk-in customers, I wouldn't risk being turned away if they are particularly busy. Remember that you can test the day before or day-of; There is no 24 hours before requirement as of today.
My experience: I went into the DAM Clinic and they gave me a fresh mask and confirmed my info, then I sat for about 5 minutes. The clinician called me in and I did the usual drill (so to speak) with the nose swab. If they get a positive result, they would prefer to do a second test, so they ask you to hang around for 15 minutes, or just go grab coffee nearby and pop your head back in after 15-20 minutes. They can also call you, but only if you have a UK mobile number. Easier just to hang around and wait for the all-clear.
Test results come in the form of a PDF file sent to your email address, which you can print at your local lodging. I fly United, and they ask me to upload it to their website so I'm "Travel Ready" -- easy peasy.
I hope this saves folks a ton of time and expense. Best of luck, and happy travels!
Michael
San Carlos, California