I'm a UK citizen and travel to the US for tourism. I've been singled out, last time was at Newark when the guy couldn't understand that I just like to travel as a hobby and was quizzing me about previous trips I'd been on.
I think I was annoying him by going into too much detail but didn't want to seem evasive either.
He ended up closing his desk and walked me through to customs to inspect my checked bag with employees there. He realised then that he hadn't properly stamped me in to the US and I shouldn't be in that area - I could hear the conversation. I was asked what I thought was so funny, the answer to which was I'm glad I took a piss on the flight before landing. They were not amused.
Meanwhile, the customs agents wanted to see in my bag but wouldn't let me go to the carousel to get it. They relented when I asked how they'd know which was mine if I couldn't go there
The trip before that I got through immigration at JFK quickly, collected my bag and one customs guy said "get him" to his colleague and I was searched for a few minutes there.
It comes across as really unprofessional a lot of times, officers can seem to possess more authority than intelligence and it irks me that - conversely - US citizens can visit the UK with just a scan of their passport on a machine of an ESTA equivalent.
​​​Doubtlessly, others will have it worse..
​​​​

When comparing US CBP at airports of entry to their equivalent at airports in other OECD countries, US CBP seems to be worse with making what I consider to be unprofessional questioning/comments -- including commentary/questions exhibiting racist and sexist biases of the passport checker -- being directed at US passport users than I've observed with their foreign equivalents at OECD country airports interacting with US passport users. Even Canada's CBSA passport checkers at airports of entry are better in that regard.

The UKBF can sometimes be pretty annoying too toward some US passport users entering the UK -- I have had my own slowdowns at LHR on some evenings because of some UKBF shenanigans at times -- but it's less common that UKBF think it's ok to cross openly into explicitly racist commentary/questioning on the job directed at US passport users than I've seen with US CBP. US DHS has a serious problem with holding employees accountable. People who can't stick to interacting professionally with travelers should be fired or reassigned to isolated duties where they can moan until they move on -- but accountability is not part of the US DHS culture.

wow, with friends like you....

Agreed on DHS - they are shambolic in that regard.

I've been to the UK at least 20 times over the past 25 years. I can only remember one time when I received less than courteous, professional treatment from the Border Force officers. On two occasions I can recall, I was questioned by a BF officer who turned out to be a Celtic supporter (once at LHR, another time at EDI) (I am a US-based Celtic fan. On another occasion, I was interviewed by a female officer, and when she found out that I am an attorney, she started asking me questions about divorce law, which I could not answer because I don't practice law in the UK and I don't handle such cases in the USA. She was quite nice actually and I wished I could have given her some insight. She let me through fairly quickly.

The only time I had any issue with a Border Force officer was at Portsmouth harbor (or should I say harbour), where I had just gotten off the high speed catamaran ferry from Ouistreham in Normandy. When he saw my USA passport he made me go to the end of the line, and then demanded to see my air ticket back to the USA; he made me miss the shuttle bus to the railway station.

The situation for USA passport holders at LHR has improved markedly, as the UK now allows us to use the e-gates. I landed at LHR 3 months ago, there was almost no line in the immigration hall (in contrast to the 90 minute minimum waits common prior to COVID), and my wife and I were enjoying the comforts of the Virgin Atlantic Revivals lounge not even 30 minutes after our plane parked at the gate in T3.

The Canadian border police seem to have an attitude of not liking Americans, although the last time I entered Canada (by car at the Blue Water Bridge in Sarnia, Ontario) in May, 2022, the policeman actually seemed glad to see me, as Canada had only recently ended its lockdown.

I've had pretty good luck with the US CBP. In 2008-2009 I flew home via JFK and actually had the same officer interview me, learned that I lived in Annapolis, MD at the time, and told me about his son's hockey tournament in Dahlgren Hall at the US Naval Academy. In Atlanta, I was interviewed by the nephew of 1960s major league baseball pitcher Billy Monbouquette.

The worst experience I had with a CBP man was about 10 yers ago at the international bridge in Sault Ste. Marie, Michigan. Growing up in western Michigan, I was always intrigued by "the Soo", about 250 miles north of my home town, which are twin towns separated by a wide river which forms the USA-Canada border. The Soo, Ontario is by far the bigger of the two cities. I overnighted in the Soo, Michigan and crossed into Canada uneventfully early on a Saturday morning. Sadly, despite being a city of 80,000 population, other than the world's most profitable steel mill and a monument to a Canadian female astronaut (Roberta Bondar), there is not much to see on the Canadian side. After about 2 hours I drove back to the US. I was rudely questioned by a CBP man, who clearly thought I was smuggling drugs or something else. He could have sent me to secondary inspection, but he insisted on berating me, figuring that he was going make a big bust. He demanded that I roll down all my car windows and open the trunk latch. He was visibly upset that other than my camera and my wallet and keys I had nothing on me (I had left everything else back at my motel). He then demanded that I prove to him that I was an attorney in Washington, DC, and I gave him a business card., Finally, he let me go.