True urban explorers don't 'break in', take nothing, damage nothing, leave nothing.
This is one i go on
Broken Britain - Urban Exploration Forum - :: Index
It isn't necessary to gain unauthorised entry to explore. It's more fun if you aren't chaperoned about and can explore freely and take pics freely but isn't necessary.
It's not illegal, it's a civil matter. Unless damage is done the police don't want to get involved.
Urbex takes photos of building that will or have been demolished, some of which won't be recorded by proper historians. Luckily the ones here that I refer to will be. We are lucky in that we probably have more listed buildings here than any other town of it's size and what we have lost is carefully recorded. Some towns aren't as lucky.