While a variety of companies carry maternity clothes, the only ones in my town (Inverness) that stock them are Mamas and Papas and H&M.
M&P was is very expensive with a small selection and the work trousers I got from them never quite fit right. That said, their tights are amazingly comfortable with the best bump-gusset I've found. In my local shop at least, they only came in grey and plum though.
H&M's maternity range only goes up to an "18" which normally would work for me, I tend to wear a UK 16-18 depending on the brand, but their 18 is really small and I couldn't get anything on. They also had a sign in their maternity department suggesting you buy your standard size when the bump is small and go up two sizes over the course of your pregnancy which is NOT standard pregnancy-sizing. Going up sizes means the shoulders should be broader, the arm-cycles cut differently, sleeve lengths change... Bah.
Ordering online, NEXT was so frustrating that when their system ate my order just as I was getting ready to check out, that I threw my hands up in frustration and quit. M&S had a very limited selection, mostly only available in S,M,L and the stuff I ordered was mostly tatty so I only kept one shirt.
With New Look I hit gold. I ordered a couple sizes/lengths of jeans, some jumpers/sweaters, tshirts, and vests/camisoles and kept almost everything. I returned the jeans that didn't fit and one "red" camisole that turned out to be a shade of coral - not my colour at all. That's pretty much what I've been living in for the last 4 month, plus a few things from my "fat" closet like a double breasted peacoat that I can only do the top and sometimes bottom button on.
With ordering things online, if you don't get free shipping, see if you can order to a local store. Try it on then and there and return anything you don't like. Good luck.
Oh, and thrift shops are a great place to get baby clothes. Babies grow out of things before they wear through them so they're usually as good as new and you can get a half dozen outfits in each preemie/newborn/0-3months and when baby shows up you can buy whichever size actually fits.