I'm one of the older residents of this country, I am a Scot living in England for many years and may be able to offer advice from a different point of view.
To answer you questions, like everyone else I start with the re-assurance that there is no hostility to Americans, if you meet a fanatical moslem he might make his point of view known to you , but generally even the few of them around a couple of parts of London are not argumentative, just strongly opinionated.
You will fit into the scene anywhere if you wear jeans and a shirt with a fleece or similar waterproof jacket, everyone here does that.
Weather, that is something we can't say too much about except that it is constantly variable...lol we can offer better advice if we know your home state then we can compare seasonal information, basically it will be warm, mild, occasional rain and wind, a bit colder in the north.
If you are using the Youth Hostel Association accommodation they have a booking forweard system where you can book on to the next hostel that you are travelling to, even in high summer this seems to work well.
Football matches are just great fun to be at, and as a visitor to the country you ar more likely to have everyone trying to make sure you understand the play and enjoy the game, maybe even a bit of hospitality after, support the local team, get in to their area of the ground, chat to people about you and enjoy it.
Food, well it's different wherever you go, just like at home, read the menu, ask for an explanation if you want to know more before you order, we all do wherever we travel. Many of the Chinese,Indian, Thai, Greek, Italian and restaurants from other countries serve up their own special dishes that can be worth trying, in the US many of these restaurant have changed their recippies to suit the American palate, here you are likely to taste the food as close to their home cooking as possible.
Few English pubsnow serve warm room temperature beer,mostly beers and lagers are chilled, ask if you are in doubt, and be aware of the alcohol content. Always keep your drink in clear sight in a pub or night club, there is a small risk of someone spiking your drinks and then you loose your wallet and all it's contents, or worse.
For places to go I leave the clubbing scene to my younger colleagues to tell you about.
A backpack is preferable to a suitacase and rolling clothes keeps the reasonably wrinkle free.
I hope this all helps and there are some places you can check out about sight-seeing I will just give you the web addresses and you can have fun looking them up.
A very cheap hostel right in the tourist west end of London easy to get to is on http://www.piccadillybackpackers.com/
for tours in Scotland that are easily afordable http://www.scotland-flavour.co.uk/ or http://www.timberbush-tours.co.uk/tours.htm http://www.haggisadventures.com/HaggisAdventures/default.htm or
Coach Travel is by National Express coaches http://www.nationalexpress.com/p.cfm?n=pg-cus
The Lake District is a very attractive area with hostel one easy day walk in any direction http://www.yha.org.uk/Stay_at_YHA/Regional/The_Lake_District.html
We have a new travellers planning guide that might help you plan how to get from one place to another and what transport to use to do that, see http://www.traveline.org.uk/
and as a guide to sight seeing in London http://www.visitlondon.com/
one thing that you can do is take a trip on a open topped bus round London, you get a hop-on, hop-off ticket and you can get off, go visit a place of interest and then get on another bus and do this all day, the ticket is for 24 hours, included is a short trip down the river.
I hope this helps you have fun planning your visit and you can contact me direct with a message if you want to do that.