Surprisingly there are a lot of stores that do Maternity clothes, however they don't make it evident when you're in the stores as to
where the maternity clothes are and staff are usually less than helpful at times.
Personally I don't really shop at Next for clothes for myself, so I'm no use in that department. But I can say their baby sleepsuits and bodysuits are fantastic value for money as they're cut longer than most so last well past their intended age range.
H&M do a good range and separate their Maternity clothes out from the regular stuff - they only do the maternity range in the UK & Europe, so you won't be able to get an idea for their stuff from the US (or online) unfortunately. They do nice dresses too, that can last throughout your entire pregnancy. I've got a fab plum coloured dress that I bought at about 20wks that's super comfy and chic for about £30...
New Look Maternity - they're consistently good at having directories in their stores as to where ranges are. Personally, I find their clothes sizes tend to run more similarly to US sizes, so you don't need to go up 1-2+ sizes to get a fit.
Marks & Spencer Maternity
Debenhams MaternityJoJo Maman Bebe do good baby clothes. I've had a few pieces off them from my last pregnancy & BF and must say their sizes definitely run slightly smaller than US & UK sizes.
La Redoute I've never ordered from them, but their maternity clothes do look really nice.
Bump to 3 - Maternity The official retailer for Grobag. I've never ordered from their maternity range, but their sleepsuits are fantastic as they're basically non-slip socks sewn onto the bottom instead of fitted feet to allow for faster foot growth.
Blooming MarvellousPumpkin Patch - Maternity They're primarily a children's clothing store, with a limited (in my experience) maternity range, but still a somewhat decent selection.
Mamas & Papas - Maternity They do really nice clothes and have a selection of both maternity and BF.
George at ASDA Maternity - a small, but somewhat decent selection and not available in all ASDA stores (check online before you go to avoid disappointment especially if you have more than one ASDA in your area).
Hope this helps a bit.