Most supermarkets have a reduced section - my local one has one section for chilled items, and another for ambient ones (they don't need to be chilled).
Some days it's a frugalist's dream, other days it's somewhat sparse. Best thing is to note which day you are in the store, and what's available - you will often find that the 'reductions lady' only works part-time, and therefore the goodies will be there on, say, Tuesday, Wednesday and Thursday.
From experience, there is little likelihood of good reductions on a Friday afternoon or Saturday. The reason is that most stores can sell it all without reducing it, such is the need.
A few examples of 'goodies' I have availed myself of:
10p for a jar of mincemeat (at Easter, with a sell-date of Easter 09)
10p for a jar of cranberry sauce (as above)
10p for a jar of redcurrent jelly (as above)
75p for a 400g box of chocolates with Valentine's Day on the box (yet the sell-by date was 10 months away)
etc
Don't expect good reductions in alcohol - the tax/duty is too high for the store to reduce it by any significant amount.
Veggies and fruit are often double-reduced. The first reduction is not that great (say 20%), and sits on the shelf until about 5pm. Then the second reduction is done, and this reduces things down to silly prices, such as 20p and 50p.
The above items were at Tes*o.
The other day at Waitr*se I was absolutely laden (only picked up a basket not a trolley) with items which were ALL reduced to 29p. And as any avid Waitr*se shopper will confirm, the food is fresher and is not run up against it's end-of-life date as in certain other stores. Managed to stagger to the checkout with 6 packs of ready-to-eat pears (kept in fridge they lasted a further 3 weeks), apples, lemons, oranges, Jersey Royals, red cabbage, tomatoes, etc. In fact, anything that I can't or don't grow myself. And it tasted absolutely delicious! Yummy, yummy, yummy.