Hi - congrats. I had DC1 in London and DC2 in Washington, DC. I think I found it harder than friends who've had both babies in the US as I ended up missing my comfort zone of M&S and John Lewis staples the second time round...
My top tips:
Maternity pads - can't be found anywhere! They give them to you in hospital but I couldn't find them to buy like you can buy them in Boots or Sainsburys back home.
Nappy bags - 100 for 30p or something in Tesco. Here they all seem to be 'premium' Munchkin or Arm and Hammer ones - 30 for $6 or something. Definitely stock up back home!
Sainsburys baby grows - in fact, we always stock up on Tu children's clothes. Every time DC1 wears them here, we get loads of compliments. Ditto John Lewis.
I can't really think of anything else. The good news is that most stuff is lots cheaper here - prams definitely, huge savings to be had. Join Zulily and 'like' brands like Uppababy and Bugaboo and chances are, you'll be able to get a half price travel system before baby arrives. Also, diapers.com is great for next day delivery on all things baby.
With DC2, I've 'gone native' on most other things as it gets too difficult to stock up on Sudacrem or Bonjela or whatever from the UK. With DC1, who was 9 months when we moved here, I tried to continue with eg the same nappy cream. Remember that a nappy bin is now a diaper pail if you're searching on Amazon. Ditto nappy rash cream is often referred to as butt paste(!). Calpol = Tylenol, Calprofen = children's Advil.
Tens machines are pretty unheard of here...
M&S do online delivery for free if you spend over £30 - always a good backup!
I can't think of much else right now but feel free to PM me. Also, if you're nearby, get in touch. There are a few of us in DC!