Going in the week is always the best idea - we went last weekend and it was crazy.
M&S do quite a good range, including in the Autograph section (it is hard to find though - it is near the tills in the corner where the Autograph and underwear sections meet) and some of their non-pregnancy range is good too - they've started doing longer vests etc.
Zara's selection is disappointing - usually just a rail but they have some great floaty tops at the moment which fit over bumps.
Mothercare is worth a look - they do some great jeans - all I've been able to wear the past few weeks. I would say though that it is worth trying them on - their bump bands come in all different heights and the mid-tummy ones might be okay for the early stages but God do they hurt later on. I actually cut mine off a few months back (I was at work, it was hot, they hurt - I snapped and chopped at them)
Dorothy Perkins is good for trousers and stuff for the sunshine and cheap. I bought 5 pairs of linen trousers & some vests all from their maternity range and spent less that 100 quid. Only problem is trousers fall down a little bit but maybe because I am so big now - Next linen ones are a fiver or so more but better quality & very cool!
The H&M range is at the back of the shop on the 2nd floor. It pays to spend a while having a look through as everything gets tangled up! Their maternity vests are brilliant - I bought 5 of the black ones and have worn them constantly as they go under things and cover the bump nicely.
Mamas and Papas (isn't one in Bluewater sadly) always seem to have sales on and do some great trousers which fit all the way over the bump. I love them - they made me feel very pregnant and proud. Great work stuff too.
Topshop on the ground floor have a maternity range towards the back of the shop - good jeans (with elasticated sides not bumps bands so great for early days and post pregnancy).
Other place I'd reccomend is Isabella Oliver on line. It is dear but I bought some work stuff from there and felt much more confortable and professional once I had - I should've invested sooner if anything. Their wrap around stuff takes a bit of getting used to though and I can imagine that on a day like today having masses of bandag-esque layering tied around you might not be that comfy?
Finally it might be worth looking on line first and having an idea of what each store has to offer before you go.
Happy shopping!