The "experts" will always tell you to make a bee-line for charity shops in well-to-do areas but in my opinion that's misguided, do so and expect stiff competition and higher prices as the well-heeled volunteers know their labels. It's better to head for more down-at-heel, less trendy areas where the locals buy second-hand from need rather than style, where practicality is prized over glamour so prices are low and pickings rich.
Earlier today I picked up this 1950's "Emgee, London" suit for £4 and the TopShop t-bar shoes for £2.95.
Never worry about sizing in charity shops, pick out what you like and try it on. Items are often incorrectly labelled and due to our ever-expanding waistlines a vintage size 12 is the equivalent of our modern UK size 8. My suit was marked by the volunteer as a Size 6, but fits like a glove and as the shoes are leather and pre-worn I got away with a size smaller than usual.