Travelling around London is actually really simple once you get to grips with the tube (No I'm not going to call it a Subway, that's a dumb name!)
Honestly not sure how much of this is common sense and how much of it is weird English behaviour, so sorry if I'm telling you things which are obvious!
Take a Walk
While the tube is great and it gets you everywhere easily, if you don't have far to go I really do suggest you walk. London doesn't have specific areas which are Old or Historic as such, it's more a weird patchwork of buildings that have been added to over centuries. Pretty much every street you walk down will probably have at least one building older than America, so if you have the time, and it's not far, go on foot and enjoy the views.
I'm assuming you all went ahead and downloaded an App for the Tube Maps, but there's a PDF here too just in case. It may look somewhat chaotic, but I promise it does make sense.
So, the grey shading in the back of the map shows the "Zones", these are relevant for ticket costs (More on that below)
As you can see, all of the different lines are colour coded. This is the same with all the signage in the Stations.
Assuming you're all able to read signs and tell red from blue, you shouldn't have any trouble finding the right platform. The only tricky bit is that some lines split, and there will be different trains going along each branch. In those cases, check the electronic signs on the platform itself to make sure you get on the right train.
General Tube Stuff
The NEgatives
Look, London is not the big awful pit of crime and villainy that some places want to make it out to be, but it is a large city with a lot of people, and that's always going to include a fair few arseholes. So make sure your valuables are in safe pockets, pickpocketing is definitely a thing around tube stations. ​
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There WILL be crazy homeless people on the tube. Not like violent/scary crazy, but definitely "Oh that guy should absolutely be medicated" crazy. Some of them may be wandering up and down handing out leaflets telling you why you should give them money. Honestly, just ignore them. It's the English thing to do.
Backpacks
If you're carrying a backpack with you on the tube and the train is full, then you definitely want to take it off your back and hold it at leg height. Taking up additional space with a bag when people are getting squished will earn you so many glares and everyone will hate you.
Fans
I know I mentioned it in the chat but bring hand fans with you. The tube is like a fucking sauna at this time of year. Only one line has proper AC (Elizabeth Line, always use Elizabeth Line when possible it's the newest nicest tube line and it's literally an entirely different experience than every other line in the city), the rest are just a big metal tube, surrounded by miles of concrete, filled with hundreds of peoples body heat.
tube Tickets
Tap on and Off
All of the underground stations now have tap on and off. So you just tap your contactless debit/credit card as you enter and leave each station.
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For Cam you'll need to buy an Oyster Card (These are available at most of the automatic Ticket Machines) And take it to one of the Customer Service Desks so that they can register it as a Young Persons Card and then he will get discounted travel. Oyster Cards are basically a prepaid card which you tap on and off with, and then you can top them up with more money as and when needed at the ticket machines.
Single fares
There are single fare prices for each journey, I'll be honest I pay exactly zero attention to what they are because I just assume I'm going to hit the Daily Cap each day
Daily Fare Caps
Each day there's a cap on the maximum you will be charged regardless of how many journeys you take. This is the number I tend to budget for. This varies depending on which zones you're travelling in:
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Zones 1-2 - £8.90
Zones 1-3 - £10.50
Zones 1-4 - £12.80
Zones 1-5 - £15.30
Zones 1-6 - £16.30
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You just tap on and off all day and then you're charged at midnight.
London Buses
No. Absolutely not. Do not get on a bus.
If you want to ride on one of the cool big red london buses, then by all means do one of the Open Topped London Bus Tours.
Do not, and I can't stress this enough, get on a standard London Bus to get somewhere.
They're unreliable, the drivers are fucking psychos, the passengers aren't much better, just don't do it!
Black Cabs
Please see above Paragraph re: Buses and NO!
Honestly, I know it's a tourist thing to get a Black Cab, and if you absolutely must then go ahead, but be warned. They're ludicrously expensive. The drivers are crazy, and there isn't a single instance where this would make more sense than getting an Uber.