Food and Drink
Eating and Drinking in London is EXPENSIVE! And while I realise you're going to want to try all the classic british grub, you're also going to need a few cheaper options that don't require you to remortgage your house.
The Humble Meal Deal
The Meal Deal is an English staple. The perfect way to grab a cheap lunch, even in stupidly expensive places like London. There are a variety of shops that offer them, and they can vary in price depending on the fanciness of the items included, but you're generally looking at around £5 for a Main, a Snack, and a Drink.
You'll find these fabled deals at:
Sainsbury's
Tesco
Asda
Boots
and probably a bunch of other places I'm forgetting, but that's a start anyway!
Be warned, we take our meal deals very seriously, you will absolutely be judged for your choices by the cashier! (No really, there's literally a Sub Reddit for rating peoples meal deals)
Wetherspoons
Look, Spoons are not nice pubs, although they will probably look oddly grand to an untrained American/Canadian eye. You aren't going to find great food here. It's the most basic, quick, pub grub ever, heated up in the kitchen by some stoned 19 year old called Kevin, but it's cheap, and it's consistent in it's utter averageness.
Plus, if I'm being really honest, this is the genuine authentic English experience. These are the places we go when we're hungover on a Sunday and in desperate need of Bacon and Hair of the Dog. These are the places we go when we want to focus on the sheer volume of alcohol we can consume in one sitting. Spoons is cheap, and a bit crap, but by god it's English and we love it despite it's obvious flaws, because there's literally not anywhere else in London you would ever be able to buy an entire pitcher of cocktails for £15. Highly recommend you download the Wetherspoons App if your American phones will allow it. It allows you to order all your food and drink from your table (And who doesn't love to avoid all unnecessary human interactions?!), plus it will tell you exactly where your nearest spoons is.
Tipping
I realise this is going to be a very strange concept for you guys to grasp, but tipping is entirely different over here. Contrary to popular belief we DO tip, but it's an entirely different thing than it is for you. Firstly, and I cannot stress this enough, our servers are paid enough money to live on! Don't get me wrong, it's still largely a minimum wage role over here, but that means they're getting at least £12.70 per hour. They absolutely do not rely on tips to pay their bills. Secondly, generally speaking the only people who we tip are servers in sit down restaurants and pubs with table service. There will very rarely be a tip jar on a bar for the bartenders, but that's not common. Thirdly, the average tip over here is around 10%, and even that's probably generous to a lot of English folk. Nobody is going to expect you to tip more than that. Finally, and most importantly, we ONLY TIP FOR REALLY GOOD SERVICE. Honestly, if your server is crap DO NOT TIP THEM. If they're alright but make no real effort to be nice to you DO NOT TIP THEM. It really is a reward for effort system here, if you have an amazing server who is attentive and friendly and awesome, then go for it, but if not then resist the american urge to give them your money!
Table Service in Pubs
This is not a thing here. I mean, there are a few exceptions to that rule, the cocktail bar we're going to being one of them, and a few places now have QR codes at the table so you can order on an app and have your stuff brought to you, but the majority of english pubs you go to the bar and get your drinks yourself. Also, places don't generally run tabs here either. You just pay for each drink as you buy it. I'm sure there are probably exceptions to that rule in the fancier parts of London (again, the cocktail bar we're going to runs a tab for you), but 99% of places don't.
Also, when you go to the bar to get your drink, don't form a line (the photo next to this is exactly what to avoid) You fill in any available space at the bar, make a mental note of who was there before you, and if they try to serve you before them you send the barman over to whoever was there first. This is the way!