Don't go to restaurants, it's disgusting.  There's no packaging or bagging really so even if they throw out good food it gets covered in gross food.

I don't really know what grocery stores there are in the UK, but in the US (California), Trader Joes (sort of a health-food/yuppie grocery place) throws out stuff 3 days before the expiration date, and also anything that is dented or has a slight visual imperfection.  It's pretty ridiculous the amount of non-gross food they throw out; on any given night we usually fill a van with no back seats.  There's usually a shit load of eggs and bread, and also a large amount of assorted packaged or clean vegetables and fruits.  One time we even found several cases of beer.

Most grocery stores donate this stuff so you're going to have to find the ones around you that don't.  Even though the ones that do donate though, don't usually do so on every day of the week so if you find out which day they don't then you're set.

Avoid any restuarant that is known to have rodent problems, because one of the methods of dealing with it is lacing everything they throw out with poison. Also when it is not cold out the place will swarm with flies and maggots, and cross contamination is inevitable so don't expect to not get sick. But if you want to find the good stuff, like the good scraper that you are, then I suggest just walking around everywhere and checking out all the garbage areas as casually as possible for anything and everything. Targeting specific places will make you suspicious and expected.

Biggest thing though is you need to have a good eye for quality, and be aware there are people who make a living picking apart anything thats worth money. Especially here in Canada where people sift through everyones garbage for metal, which is called scrapping for some reason, and go to certain government facilities that give you money for bringing the stuff over.

But really don't raid bins for food, it is the least rewarding and most likely to get you sick. The smell also tends to cling to you for days if not weeks and you'll find yourself doing alot more bathing then you do now.

EDIT: Also, clean up after yourself. People will not turn a blind eye if you get caught, but if you clean up after yourself then the punishment for getting caught would be alot because they wouldn't have to clean up after you, which is what gets to the employess/people the most.

Your best bet is to work with some Binsider Binformation.  Talk to the employees and they can probably tell you what day of the week/month that the shop Bins old merchandise.  My friend would dive at Blockbuster and find trashbags full of decent movies and games.  It's also a well-known fact that Krispy Kreme tosses out their day old donuts at 11pm.  

Also, watch out for Bin Bums.  Depending on where you live, you may be crossing into bum territory, scraping with those that make a living off of Bins.  We call these Guardians of the Bin.  They are highly territorial, so watch out for any Bin marked with hobo piss.

Not everyone does it as a conscious choice, some people do it because it was either that or stealing.

Anyways watch out for the cockroaches and rodents, kill one they'll run off and come back a short while later. Anything at the bottom of a dumpster should be considered bad. Green (in Canada atleast) dumpsters is not where any garbage is thrown, but is where card board boxes are thrown specifically. Be sure to know when employees go in and out of the facility so you can time it. Your better off stealing the damn food, I can't stress this enough, just dumpster dive for other things besides food. And if anyone catches you any respect they'll have for you will be gone. Lastly watch out for cameras.