Its really annoying, I couldnt believe that most high street stores dont stock maternity clothes anymore. I spent a good 30-40 minutes walking the length of Oxford Street about a month ago in search of maternity clothes and they all said check online. Im 5ft 7'' and a size 8-10, so a lot of the online stores are difficult as they always come up really big and long (made for ladies at least 6 ft tall I think?)
I did, however, stumble upon quite a nice range in Topshop at Oxford Circus - well worth a look!
You could also try the larger H&M's (and sometimes the H&M kids) - They do a range called MAMA. I bought some great t-shirts (some with buttons down the middle, so great for potential nursing later on too) and I can really reccomend their tights - no baggy bits at the ankle, really comfortable. I bought these bits from their High Street Ken store. BTW: Their leggings come up really long on me, so be careful of some of their sizing.
I got some very comfortable jeans from Gap on High Street Ken too. Again, the sizing is quite generous, so best to try these on first.
Im fast growing out of warm cardigans and woollens but I did spy some really good longer length ones in Uniqlo that would probably do most ladies fine through maternity.
I have also heard good things about New Look 'Mama-licious' range. This is online, but the sizing is supposed to be as expected. They do a free store pick up service, which is quite handy if you have one nearby.