I'm afraid it's not good news.
No sign of maternity in the Brighton H&M.
In terms of high street shops, I think only Dorothy Perkins has a mat range in store in Brighton.
Tho actually, United Colours of Benetton has a small selection.
and almost next door, American Apparel does lots of stuff that could be worn in pg, lots of long stretchy tops store.americanapparel.co.uk/women.html
Other than that:
There is a Jo Jo Maman Bebe in the North Lanes
and a new (and lovely looking but expensive) shop is opening there too - called Mamissima I think www.mamissima.co.uk supposed to be open in Aug 08 but the workmen are still there with their hammers.
And there is a place in Hove called yummy mummy maternity www.yummymummymaternity.co.uk which is ok but a bit expensive.
So, there are boutiquey places, but the high street is a bit pants.
Having said that, I've surprised myself at coming up with so many.. if only I had money, I'd go shopping
Oh yes - and before I forget, apparently Gap in Tunbridge Wells carries the maternity range. Not TOO far away..
Hope that helps!