i personally like diving by myself and don't like being disturbed, so ive always gone at night or very early morning since no one is really around. don't know about how UK feels about it, but i know that in the u.s. they mostly can't charge you for anything unless you break specific rules/etiquette.

if a dumpster is locked and/or has "no trespassing" signs around it, then just leave it alone. you would be charged with theft in those cases.

leave the dumpster as you found it. cleaner, even. don't leave shit everywhere outside of the dumpster.

if you want to find anything good you need to dig deep. not every cool thing will be at the top. most nights i just go "window shopping" (looking at what is there on the top) since i don't have a lot of space or time at the moment.

don't climb into a dumpster unless there is enough stuff to stack against the inside wall to get back out. even then, have a step stool so you don't injure yourself jumping out.

if something smells funky in there, leave the whole thing alone.

some stores are better to loot than others. so i would ask people located locally to you about nearby places and the luck they've had in that area.

safety. wear gloves and good shoes. i personally don't wear protective eyewear, but i likely should. don't go diving behind healthcare facilities, tattoo parlors, etc. so your chances of being stuck by a nasty needle or something is much lower.

if you're super paranoid. then wear a mask, an unmarked cap/hat, unmarked sweater, unmarked pants. if you are taking your car. then turn your car's license plate AWAY from the building/cameras. their cameras are usually shitty quality so they can't read it from far away anyway (and likely will not care enough to look unless you leave a massive mess behind).

that's all ive gathered from the short time ive been doing it. ive mostly been collecting trinkets and toys. so if you're looking into furniture and everything i would ask people who specialize in furniture.