What should you take when you go urbexing?
The basics: a torch, camera, friend, water
If you're a bit more serious: rope, climbing gear, helmet, high vis, small first aid kit, 2 or more torches, high-vis, tripod, friends, food, water all contained in a nice sturdy backpack.
You're of course going to need the most basic of things: a torch
Urbex in most places is pretty difficult at the best of times, and without a torch you might not notice a hole in a floor, miss a wall or just get lost. The places you visit don't have lighting, it's all what comes through the windows (which might not be alot). You should also carry a backup, incase your main ones battery dies or just stops working cos you got it out your basement and threw a few Energizers you saw on your desk into it and ran out into the first derlict building you saw.
So, you need a decent torch or two.
An expensive, but brilliant torch comes in the Lenser series.
The Lenser MT7 has 220 lumens of illuminating power to guide you through whatever you find, and it £60, maybe a bit much for the relaxed explorer.
The torch most of you would buy is the Maplin CREE LED torch
You can pick one of these up for £15 from your local maplin and they are brilliant. You can buy a couple of em, stick them in your bag and go out. According to one review, you can even batter people with it if they get too close for comfort.
That's just one of the things you need. If anybody else wants to write a section on a bit of gear they use (tripods + cameras is a good place to start) post it and ill chuck it in the op.
Something else you might consider packing is a high-vis jacket. If you're sneaking into somewhere near a building site or some railway property, wearing a yellow or orange high vis always helps and people will see you and not really care you're there. I pack a plain yellow one and an EWS one I acquired legitimately.
You might also want to get a cheap first aid kit, they usually can be found in pound stores, just incase you fuck your hand up on something it would let you patch up and make going on a little bit more bearable.