I am a HUGE fan of M&S at the moment. They have a lot of bigger sizes (gweilo cuts, too) and trousers with elasticated waists. They also have a lot of wrap around dresses or stretchy stuff that look really flattering. I have just bought a whole wardrobe for work from there and am looking forward to going back to get more casual stuff once maternity leave kicks in. It is also a good place to go post-baby for new clothes.
I was also surprised at the stuff they had in Zara that is good in pregnancy. It seems that the pregnancy look is in these days.
I haven't checked out H&M but did go to Bossini on QRC. Not that impressed, to be honest, as the range was quite limited and I don't think I would have fitted into them.
The other thing to rate about M&S is they have started selling maternity bras here (I know this is new as they didn't have them when I was expecting 2 years ago). They have some pretty padded maternity bras that I just love, as well as the softer types and nursing bras. They are quite flattering and very comfortable.