A310driver
As a youngster I was pro-American (Hollywood propaganda, pleasant & well behaved visiting US forces and, in my 20s, visits to the USA.
As one becomes older one reads and considers history, classified government documents are released and one understands more clearly the political reasons behind decisions made over 60 years ago.
During WW2, the 50 or so old destroyers sold by the USA to the UK were very welcome in their anti-submarine role helping to run the blockade. Getting supplies across the Atlantic was, at that time, the most vital contribution to British freedom. When Germany failed to gain air superiority over Britain followed by reverses in their war with the USSR it was only going to be a matter of time before they would have to sue for peace but unconditional surrender would not have been offered.
The US contributed greatly to the air effort and without US forces an invasion would not have been possible. No-one is unaware of the huge losses incurred by the US in these operations and their sacrifice is remembered and honoured.
Although previous terrorist/nutter events were perhaps viewed by those who believed in American support of the IRA with a slight degree of Schadenfreude, the the events of 9/11 were watched with open-mouthed horror.
Now the negative bit:
The US was somewhat miffed when the Brits returned to Singapore & Hong Kong after WW2.
The US effectively threw the Brits out of Suez using economic pressure.
The US considered invasion of several Middle Eastern countries including Saudi Arabia in 1973 (Who thinks Iraq isn't about oil?)
I believe that the UK is only now making the final payments to the US for debts incurred during WW2 (Sterling should harden on this demonstration of reliability)
An oversimplification? Yes, grossly! - but whilst we admire much about the USA, particularly the freedom of information and speech which permits us to see warts and all, don't expect blind love.
What you interpret as anti-American may be no more than healthy open-eyed debate. Brits sometimes have difficulty seeing the US as a foreign country and therefore can be as forthright in their opinions about your country as you would be yourself.