I don't know of anything with discounts of the scale of the sites they posted above (although I'd love to), but bookdepository.co.uk sells cheaper than brick and mortar stores tend to, and is cheaper than amazon.co.uk the majority of the time. They send stuff out fast, and shipping is free (however you can't preorder anything as stuff that isn't released yet is just listed as 'unavailable')
Abe Books is really good! You can get new and old stuff for really good prices :)
As far as I'm aware, their shipping costs depend on location, but they're usually a great deal ;) xx
I buy it off of Amazon because they usually have cheaper prices like sometimes $6.00. It's also nice because you can buy more volumes until you pay enough for free shipping which I believe is around $40.
However, I would not pay it if you were only getting one volume from one manga because shipping+handling is about $6 or $7 and tax is $2. I use gift cards.