I took loads with first baby and most of it never got out of the bag. This time around everything fits in a small wheelie carry on bag .
Don't forget your DP will have to carry your bag(s) and possibly push you around (if you are in a wheelchair) being transferred from waiting room to triage room to labour ward to delivery suite and back. Less is definitely more.
In answer to your question, real "essentials" are nappies for the baby, a few muslins, a couple of baby pyjamas, a baby hat and cardigan, blanket to wrap baby home, maternity pads, towel, and something for you to sleep in if you don't want to wear a hospital gown. Primark has some really cheap nighties that you could wear for the birth and bin afterwards. Everything else, in my opinion, is up to you - although I will also be taking a nursing bra, breast pads, cheap cotton knickers that I'll bin after use, flip flops, wet wipes (for me and baby), toiletries, and a paper fan as I got ridiculously hot last time. Oh, and camera of course.
You won't become a "different woman" the moment you give birth, so no point taking stuff you don't use at home on a daily basis. For instance I never wear a robe, so I'm not taking one with me. Never occurred to me to take clothes for DH, he couldn't spend the night anyway (hospital policy) so had to go home and could shower and shave. Ditto for snacks and drinks, I have yet to see a hospital that does not have a canteen/vending machines and frankly food was the last thing on my mind. I took my pillow last time (as advised by midwife) only to find out each bed on labour ward comes with 3 pillows so really didn't need one more. Hopsital also provides antiseptic wipes available so I don't need those either.
I'd also advise involving DH in the packing, mine didn't know where anything was (or indeed WHAT most of the stuff was!) whenever I asked for something.