Hi! DD1 was premmie and she needed clothes - think it depends if they go in an open cot fairly quickly. If you google, there's loads of sites where you can buy tiny clothes, some of which accomodate the various wires/tubes they may need. Boots and Mothercare also do nice ranges.
I've put a link below for you (not FZ endorsed). Its a site for parents who have/have had babies in NICU/SCBU - you'll find really useful info here, and lots of support.
http://www.bliss.org.uk/
Take lots of change for parking at the hospital. Some hospitals give you a free pass if you have babies in NICU/SCBU, and in others you can sometimes buy a pass that will cover you to leave your car in the car park for as long as you want whilst your babies are in the unit - its worth asking as you'll probably be there a lot and the parking charges soon rack up.
Most units have breast pumps available I think, and both the units we were in had rooms where Mums could pump or take their babies to feed once they were able.
As you already know what unit your babies will be in, you could contact them and ask what facilities are available and what would they advise you to bring in. I'm sure someone would be happy to help answer any questions/queries you have.
Wishing you lots of luck.