Hey.... just out of curosity what do the Brits see the Americans as....
Americans in England
Well most of the people have a kind of low key resentment that they won't act on I'd say. On the other hand there are some people that really like Americans and American culture (particularly older people for whatever reason). When it comes down to it though, a lot of people might dislike Americans in the UK but it's exaggerated, I know a lot of Americans that live here and have never had any problems whatsoever save the odd tongue in cheek comment. Maybe if you went into a rough looking pub on a Friday night and brought up world war 2 that might be different, but apart from that it's fine. You'll probably get annoyed with people asking you if you voted for George W Bush all the time though.
Americans can get a bad rep for being loud.
But having said that most of the people i know love them.
Just depends on the person, remember the UK has probably the biggest mix of races of people anywhere in the world.
It's the person that counts, not where they are from 
im American livin in Britain for 2 years now and have to say...whether Brits like it or not - the culture is exaclty the same.
We both have weight problems, are consumer cultures, not the most educated young people, and our two leaders are in bed together. So Brits are used to us...its the continentals that dont really like us 
and Brits are much louder than Americans any day.
It depends where you go. My advice is to be yourself and people should like you for who you are and not what accent comes out of your mouth or the country you are from. I am from Britain, I'm by no means proud of my nation but I hope that people understand that I am not responsible for the actions of my government and take me as I am.
Just so you are aware, the stereotypical view of americans is big, loud and proud with no dress sense. As long as you don't fit any of these then you'll ave a great time and folk will warm to you.
Quoting jase007
It's the person that counts, not where they are from
Speaking as an American who has visited the UK 6 times, (going back to London 21 April), I must say, I have never encountered any negativity. That being said, I have always tried to be the "traveller" as opposed to the "tourist" and be respectful of cultural differences. That means not pointing cameras into peoples faces, wanting to buddy up to someone in a pub and discuss politics or boast how the Americans somehow "saved" the UK, etc.
Funny enough, last time I was in Paris there was a Scottish gent staying at our hotel. I was working so hard on my French that I found myself speaking English to him with a French accent. (Never even noticed until he asked me where in France we lived) 
We laughed about me being American and I said, "I hope you don't hold our ridiculous politics against me."
His answer, "I won't hold your politics against you if you promise not to hold our politics against me!"
As Jase and some of the other posters have pointed out, it's the individual, not the stereotype that counts.
Ladymacwilly
Go and have a great time! (BTW, if you want to "blend in" a bit rather than screem "Yank", leave the white tennis shoes and the clothing emblazened with logos behind and don't be travelling with a suitcase the size of Texas.) 
leave the white tennis shoes and the clothing emblazened with logos
Yeah cuz u may end up gettin mistakin for neds!
That being said, I have always tried to be the "traveller" as opposed to the "tourist" and be respectful of cultural differences. That means not pointing cameras into peoples faces, wanting to buddy up to someone in a pub and discuss politics or boast how the Americans somehow "saved" the UK, etc.
Completely agree with Ladymacwil.
I have only come across two things that have annoyed me in the past about American tourists (this is generalised by the way, but have encountered it more with Americans than other foreigners). I was in a restaurant in Austria once when an American started speaking really loud, slow English and generally treated the staff like idiots because they couldn't understand him. (He also wanted to pay the bill in US dollars!!!!
).
The second one was when we were in Costa Rica talking to a group of Americans and Costa Ricans, and the Americans were really dissing Costa Rica and basically saying how amazing the States was and how undeveloped and backward Costa Rica was.
Be yourself, enjoy the place - and definitely make sure you let people know that you're enjoying the place! (everyone loves foreigners complimenting their countries!)
I'm a Brit living in the states and I agree with Jeff G. Brits are much louder than Americans :-) My family just left our house after 2 whole weeks and my American hubby bless him said my god they are all so loud :-) Might be as some are hard of hearing but ah well.
I liked the overweight comment also. Brits do like their meats and potatoes. Nice people like nice people so stay away from the idiots in the pub looking for trouble like my brother. His name is dave so stay away from anyone called Dave ha ha. Have a great time in my home country. Enjoy good fish n chips. I miss them dearly.
sorry Argente I missed your question completely. I can only say that my husband has been to the UK 3 times and never had any problems. Most Brits just look and smile as he speaks with his american accent. Mind you he's a charmer.