Don't know any particular places - trouble is, everywhere has a couple of shops but there's nowhere I know where you can get everything.
Here's my survey of shops for what it's worth:
M&S don't do maternity clothes in store at all AFAIK (shop assistant in Marble Arch said it's all online now) but you can order online, pick up at the shop or get delivered home and return.
Next do some maternity clothes instore but it's a bit hit and miss - again, you can order to get delivered to the store or toyour house - if you don't want to open an account with Next (I didn't) phone them up, and you can just pay your bill by card.
Topshop have really nice stuff - it looks like it's for an older crowd than your normal Topshop customer which was good for me! You can check their in-store stock levels online, so can save yourself a wasted trip. The Oxford Street one has a decent range of maternity gear although it's always busy. All their jeans are skinny jeans. Maybe you like that. I'll never give up my boot cuts - all I need to do is wait another 10 years and they'll be back in fashion.
Gap's OK instore, can't check store stock levels online though and they seem to have been sold out of most sizes of jeans since about November... again the Oxford Street shop's got a decent range
Debenhams Red Herring's not bad - I'm wearing a pair of their trousers right now
Mamas and Papas have posh looking clothes but like Monsoon they seem to be made for a 6 foot tall woman and they're quite baggy around the bum. Don't know about online ordering.
H&M have some bits and pieces in most stores I think
New Look's supposed to be good - I ordered a coat from them online but haven't been in any of the stores.
Also can I just throw in a rant about how, in December, I went looking for a maternity coat and all the stores were putting theirs in the sale so there was almost nothing left? OK I get that the fashion industry thinks we all buy clothes 4 months in advance of needing them, but surely they don't expect people to do that when they're pregnant? I only got past 12 weeks in November and there was no way I was tempting fate at that time to buy maternity gear, even if I'd had any clue how big it needed to be.