I work as a TA and would say that if you are not sure, pop in or give the school a call, there will be somebody in the week before they start, if not already.
From my own experience, your dd may be better going to school in her 'winter' uniform, with a polo and cardi. I would say 98% if not more of the kids, start in September in this. Unless it is really, really hot, then you may get a few in summer dresses. Your dd will stand out if she wears a summer dress all through the winter. Maybe you could get one dress and then see what the others have on, but I can almost guarantee unless it is a heatwave it will be skirts and shirts - even little ones want to be the same as everyone else on their first day, so please don't send her in a summer dress the first day!
Boys generally wear jumpers, girls cardis. ideal if your dd is a 'hot' child. They take cardis off and have their polos on. You will need a top for each day (unless you like washing), as reception are a mucky lot! bottoms can last longer, but I wouldn't count on more than two days!
The reason they say 'standard' size lunchbox, is because 30 large bags, or rucksacks, take up hell of a lot of room in class. I bet you several people will bring them in with plastic boxes inside! We try and encourage the small plastic ones with handles on (usually with a character on too), as they are stackable - too may parents send in huge bags, which the little ones find hard to carry too. Putting an icepack or frozen frube in, is to keep them cool. We cannot put lunchboxes in the fridge and they will be sitting in (what is sometimes a hot) classroom for several hours and believe me, food in plastic containers sweats!
Waterbottles can be anything, from actual bought water (cheap ones from Tesco are ideal and can be refilled), or proper ones, make sure a name is on it - black marker pen is fine. Same for any snack - write names on everything, even bananas! Water bottles are usually put in a box for the children to access throughout the day.
Forget anything else for first day, apart from lunch and water bottle - don't even take a bag unless the school have given you one. The less they come in with, the easier it is to send them home. A folder will be sent with a reading book in when they are ready, or the teacher will ask you to supply one in due course. Wellies can wait for a week or so, get a cheap pair and use a peg - write their name on wellies and peg, to keep them together. They will stay in school.
PE kit needs to be easy to put on and on PE day - your teacher will tell you which day - if not ask!! - ensure your daughter has socks on - not tights.
I would suggest first day you take PE Kit in and ask the teacher if they need it and what day it is. Usually you will get it at the end of the week to bring home and wash - although one school I was at, it only got sent home at the end of term!!!
Don't stress, all she needs is her uniform, her lunch and water bottle, along with PE Kit just in case. Just make sure everything is labled.
The first week is a steep learning curve for everyone and don't worry about asking the teacher or the TA if you are not sure - we are used to it honest!