It's a good point (and this thread is soon to be moved to the political section).
Being a Brit who has lived and worked in the US for almost half my life, I have a good perspective on both and your comments above, I've heard multiple times. It's simple. You are bringing up points on Imperialism which are historical and clearly out of date in modern times. It's akin to bringing up historical facts of American forces genocide on Native Americans, their country being built on slavery or the fact that some states practiced cannibalism, fairly recently.
The US came into the WW2 fairly late (and through being forced into it) and their has been ongoing issues with their insistence (perhaps not unjustly) about how they "saved our asses" yet made a huge amount of money out of that war and I think there is probably some issues left over from there but the real reasons why British people complain about the US politics are multiple fold.
The US has meddled and bullied (and often with the UK governments help, even though the majority of people in the UK didn't want it - and in coming elections voted against those parties) in many foreign policy disasters. Their "chicken-licken" war against Communism was puerile and going to war with Iraq (and dragging the UK into it despite most public not wanting to - thanks Tony!) under the excuse of of weapons of mass destruction was extremely damaging. I also think many in the UK are disgusted at the way subsequent UK governments have bowed to US pressure, so much of the ire is unfairly places at the feet of the US when it the UK government could have listened to it's people.
Finally, and with no disrespect intended - it's because every time we see the US on the news, you come across as a bunch of absolutely mental, crazies. I see this in the US news also. There are things that go on in the USA which just seem so outrageous, immature, out of touch and over-the-top that the rest of the world, find it hard to see the US as a credible country. From the polarization of politicians and the inability to find middle ground, from the amount of gun deaths (and the unwillingness to blame it on guns despite the statistics), from the apparent on-going racial issues/tensions, the anti-abortion lobby and the terrorism that goes with that, vote fixing and allegations of stolen elections, the madness of the health and pharmacutical industries not allowing booze to 21 yr olds but allowing them to go to war at 18, the hypocrisies on human rights, etc.
On the other hand, the UK loves (obsesses) about many US things from music and entertainment, much of the culture and we're possibly one of the larger tourist groups to visit the US. American's generally get a warm welcome in the UK. Perhaps it's because we see a warped refection of what we could become ... or what we once were.
The second greatest trick the US ever pulled, was convincing the world that they were the "greatest country on the Earth".
The greatest trick the US ever pulled was convincing their own people, that they were the "greatest country on Earth".
While most nation's people are patriotic, only the US people appear to be as blind as some dictatorship's countries. The US is the only Western country where I've ever heard people who criticized the government being called "unpatriotic" and to "leave the country if they don't like it."