I left the UK for the USA on 3 June and returned back to the UK on 21 June.
To enter the US, I used this test provider:
https://midlandhealth.co.uk/ [nofollow] . Paid for an Antigen test. Upon receipt of negative test via email, I asked that the letter specify it was a **viral** antigen test (as per CDC wording in guidelines). The request was granted. No problems entering the US.
Harder for me was figuring out which test to take to come back to the UK. I understand some airlines offer to let you take kits with you. I hadn't realised this when leaving. Not sure how or if these can be processed in the USA.
In the USA I was not in a major urban area and there were no travel test providers in person. I couldn't find any test provider that guaranteed they could meet the UK entry requirements. Read pages and pages of FDA documents, trying to get them to match the UK requirements. No luck.
Finally decided to chance it on Vault health, which is an FDA approved test provider - they use the first PCR spit test that was approved by the FDA rather than the second one, and the first one is seemingly more sensitive. I called Vault and they could not guarantee it would meet UK requirements. My test was free in the mail via Minnesota (thank you Minnesota).
https://learn.vaulthealth.com/travel-int/ [nofollow] The Vault website suggested I needed to order the test a week in advance via mail but it arrived in a few days. They do not support results in time for flights on Tuesday or Wednesday - see link above. My flight was on Sunday thank goodness. I took the test on Thursday and dropped it off at UPS before air pickup as they said. But they didn't get my test until Saturday morning, and I got an email saying the results would be 24-48 hours. In a panic I called Vault health, explained my situation, and my results were ready that day. They also apologised for the stress caused by the delay. I am very suspicious of the US post system right now as I also had a Walgreens order delayed by weeks!
No problem using the Vault letter at the UK border. I would use them again.
I had booked my Day 2 and Day 8 tests via an approved UK supplier, Randox, and these were waiting for me at home when I arrived. NHS Test and Trace called me today (Day 1) to check I had received my Day 2 test and they said they will be calling me every day. I filled out my passenger locator form online. I read the reviews on some test providers and people were saying they hadn't received their tests by the time they got back to the UK. I did order mine about a week in advance though.
Also, it was pretty much roulette at the Heathrow e-gates by the way. I would say around 1/3 got through no problem like me. But around 2/3 were told to 'see agent' and had to wait in a very, very long line and weren't let through automatically.
Good luck!!!!