I have bought and loved things from Dorothy Perkins (leggings, tunics, yoga pants - best thing ever) and H&M (dresses, jumpers, trousers).
H&M trousers I have had to go two sizes up - 12 was just a bit too tight, but 14 I have to hoike up all the time
but I do love my over the bump jeans, and my latest purchase - some under the bump skinny chinos!
I'm just coming up to six months now, and have managed to keep wearing quite a bit of my non-maternity gear - jersey dresses, peasant tops, long-line t-shirt/strappy tops... although I have to say at this point, wearing anything too loose just makes me look fat rather than pregnant, which isn't helping with the whole being offered a seat on the tube thing... 
As you don't need workwear, I wouldn't have thought you'll have much trouble finding stuff - it's the needing to be smart that is a pain in the arse for me at the moment (although I am rapidly beginning not to care). As summer approaches (I hope!), I will probably switch to roomy shirred summer dresses.
Also v lucky to have a mum who is a dab hand with a sewing machine 
On the bra front, I am also quite big up top. I bought a pack of 3 genie bras from tesco for £40, and I have to say they are the most comfy thing ever
I sleep in one every night, and have even started wearing one during the day sometimes (they come with little padded inserts if you want a bit more padding/peanut smuggling prevention). I had been soldiering on with my underwired bras, with the aid of an extender to increase the back size a bit, but the more LO keeps hopping up towards my ribcage, the more uncomfortable it is getting...