I kinda learned by trial and error what to bring with me to London Expo. The convention organizers were actually kind enough to write a "Essential things for Expo" page in the con guide...but since I didn't pick that up till I was already there, it wasn't that much help my first year!!
I'm going to write this down as a personal reminder, but hopefully others will find this helpful too!
London Expo/General Convention Essentials!
WATER. You will not believe how hot it gets in crowded convention halls, and you get dehydrated crazy-fast. At London Expo there are a few water fountains, but you will have to queue for them, and bottled water for sale on the day costs a fortune. It's a little heavy to lug around, but I always bring a minimum 2 bottles of water with me.
Dress Light, wear layers. Like I mentioned above, it gets really hot in the hall, its best to wear layers so you can take off your jacket/jumper/whatever when you start getting hot.
Money. Most, if not all, of the dealers will only accept cash. TAKE OUT CASH BEFORE YOU GET THERE. Seriously. The queue to use the cash machine is ridiculous. Think 45 minutes or so to get some cash out. And usually the machine has been emptied long before the con closes!
Check the Online Con-guide before you go. It's incredibly frustrating to find out there was a panel with actors from your favourite show that you've just missed 'cause you weren't aware of it! Have a quick flick through the guide before you go and see if theirs anything you really really wanna see, that way you can plan the rest of your day around it.
Plan your Journey.Every year I see groups of people in cosplay wandering around London underground looking completely and utterly lost. Check your route before you go, and if you are using the underground, make sure there aren't any line closures! Depending on what line has been shut, you may have to take a big diversion to get there! Also be aware that if you wait till the very end of the con to leave not only will their be massive queues to get on the trains back....you may have to negotiate London traffic during rush-hour. Even on a Saturday or Sunday...that's a BIG no-no if you can possibly avoid it.
Bumbag. No-wait, don't scroll past in disgust!!! I know bumbags are kind of a fashion faux-pas nowadays but you won't believe how handy they can be at a con!! The first two years I just brought a rucksack, and it's so crowded at the event I found it sometimes impossible to swing it down to reach my cash when I needed it (at least not without knocking someone over in the process). Having it handy on my hip is a real life-saver!
Mobile Phone. If you go with a group it is easy to get accidentally separated. Texting is a life saver. (You can try calling, but it's almost impossible to hear each other over the noise!)
Medication. Take any medication you're on that you might need. I'm asthmatic so I take my inhalers everywhere, but that first year the sheer noise of all the people at the event gave me a splitting headache within the first couple of hours. Since then I always make sure to take some asprin with me.
Tickets! Obvious, yes, but easy to forget!
Lunch! Yes, you can buy food there. But there tends to be huge queues, and once again, it's pricy. I just bring some sandwiches with me. Why spend money on food when I could be using it to buy manga!? If nothing else shove a couple of energy bars or something similar in your bag before you go, just in case.
Camera. There are spectacular cosplayers at Expo. You can use a camera phone of course, but if you're really into cosplay and wanna take some really good shots, bring a proper camera.