I’m American and have lived in England for about nine years now.
Some of the main points:
When you pay for something at the ‘till’ the price that is marked on the item is the price you pay. Tax is already figured in.
Trains and Underground stations are very easy, but might be intimidating to someone there for the first time. You can pick up an all day travel card that will permit you to ride the trains, the Underground, and the buses for the entire day. Pick up a tube map. They’re free.
The different lines are indicated on the map with different colours. There is almost always a gate person to open gates for people with suitcases, and they always know the routes. If in doubt, look helpless and ask one of them. They’re used to it and are usually always nice.
Dress in layers. It goes from one temperature extreme to another. Dress for rain unless you’re 100% sure it won’t rain, then still carry an umbrella. Wear the most comfortable shoes you own. If you’re not used to walking, you’ll feel the strain.
There is a Fitness First Gym at London Liverpool Street Station. It opens at 6 am. They’re all over London, but it’s handy to have somewhere you can go and then travel out of to get on with your day. I’m not sure what packages they offer, but you can get on the net and find out. Or as someone else suggested, lie.
I think they do give a week free trial. They’re linked to a network of Fitness First gyms all over London. They have good facilities and even a sauna.
Everything is expensive in London. Just get used to gasping at prices. The main fast food places in England are Burger King, MacDonalds, Pizza Hut, and KFC. There are really no others unless you want to include Wimpy’s. There is every food known to man in London.
Most pubs serve a good, cheap meal so you can relax, have a pint, and get a meal at almost any pub. If you need a bathroom ask for the loo or the water closet. It’s the words they use. Most of the wait staff here don’t work for tips, but being American, I can’t get out of that habit, so I tip.
Taxis. Expensive, but really cool. A small family could live in one. You might want to take one for the experience, but use the Underground (Tube) for economy. Stations are EVERYWHERE. Get a London Guide to find out which station spits you out where.
Queue - Just get in line and get used to it. No one really talks in lines in London, but if you ask questions, most Brits will be happy to help. They love Americans. They’ll tell you all about their trip to Disney. 
Fags - July 1 is the great smoke out in England. No public smoking of any kind. Until then, don’t freak out if you see everyone smoking joints. They’re not joints. Many Brits roll their own. Girls walk arm in arm here too, so don’t think the lesbians have taken over. Enjoy the show. London is a colourful place full of every diversity of style you can imagine. The warmer the weather, the more naked the girls.
The sidewalk is called the pavement, and you’re best to stay on it and follow the traffic signals when you walk, or they’ll run you down.
A liquor store is called an off- license, and a convenience store is called a news agent. You can get all your toiletries or medical needs at Boots - they’re Britain’s largest pharmacy chain and have everything you need.
WH Smith is the biggest book store, and like Boots, are all over. You can get good magazines from all over the world there. The airports and the train stations have all the above mentioned chain stores.
I’ve been all over London a zillion times. It’s fun and hectic and there’s a zillion tourists there. Just put on your back pack and blend. Be aware if you ask for directions that they may not speak English, or may be a tourist themselves. Sometimes it’s best to ask the guy who sells newspapers at the station. He usually knows everything in the vicinity.
Toilets work by gravity, so you usually have to pull a chain. Pull it several times if at first you don’t succeed. Hotels have the regular plumbing, but some pubs and restaurants have really old fashioned loos, and interior doorways that are a bit low.
Sainsbury’s is one of the biggest food chains here, and you’ll find them everywhere. You can pick up some food for your day to store in your backpack. There are plenty of fresh produce agents around, and London Liverpool Street Station has a great fruit stand so if you go to the gym there, you can stock up.
There are NO trash cans at train stations (from the old IRA bomb days) Just get used to carrying your trash until you can find a ‘rubbish bin’.
It’s safe to eat beef here now, even on the bone. It’s been my experience that you can’t find a decent steak easily. Try a ‘Sunday’ style roast dinner - it’s standard, especially in pubs. Yorkshire puddings aren’t pudding at all, but these big airy puffy rolls that you pour gravy in… they come with a traditional roast.
Fish and chips are good and you’ll find them everywhere. They use vinegar instead of ketchup on the ‘chips’. Potato chips are called ‘crisps’ here, and french fries are called ‘chips’.
If you get pizza, ask for them to cut it up. I swear, Brits will cut up an entire pizza with their knife and fork. Pizza Express is the best pizza around… chains everywhere in London. Oh, and they call crackers ‘biscuits’ so if you want a biscuit, best to ask for a scone, but it’s really a breakfast or a coffee snack item. There are Starbucks, Costa and Pret Mangers all over town if you like the coffee bar type places.
OK… think I’ve covered most of the major bases. Study the tube map and the London guide before you come over so you will be somewhat prepared. You can take the Gatwick or the Heathrow express from the airports straight into London. Easy as pie.
Find out what tube station your hotel is near and head that way. There’s little to no ice here, so don’t expect that. They also don’t have washcloths, even in the most posh hotels. They’re simply not used in Europe. Bring your own if you’re partial to using a wash cloth. Store a few in zip lock bags.
Have fun!!! If you have any more questions, feel free to ask. Sorry this is so long, but there’s so much to know!!!
Micki