Hi freetrait:
It's very tempting to go out with the list of uniform items from the school dress code and get in the lot but my advice is get a core wardrobe (a few gingham dresses, a pair of trousers (and by the way some do a thick cotton type that may be more similar to leggings) and a cardigan. Basics like socks, gym kit & gym bag, plimsoles, school bag, etc... also worth getting.
When you visit the school - have a good look at what children are wearing - for example, only boys wear sweatshirts at our school, the trend for girls is for plain cardigans or school logo fleeces.
Go for things that pull on - usually at YR managing buttons or zips can be tricky for little ones and teachers and TAs don't always have the time to help. For thin girls, go for adjustable waists where you can button in the elastic or if all elastic waist - just sew up bunches of fabric (left - middle - right) for a better fit.
Do invest in tights now (they disappear by autumn) & get at least 5 pairs, so you don't run out during the week.
Then wait and see what other children are wearing and what she likes. Catch the sales in October and stock up on the rest.
There are so many opportunities now to lay in school clothes (on-line BHS/ M&S/ & others) + many places like ASDA/ Sainsbury's/ etc... also stock uniforms.
The good news is that in Class R they aren't very picky about what they wear. As they go through primary, DCs definitely need consulting, because they will start to have very clear ideas about what they want to wear or find comfortable.
Often you can order logo items through the school or they arrange a stand at the Christmas Fair or similar - so there will be opportunities later in the year to add to the 'core' school wardrobe.
Oh and a word to the wise, because I have been caught out, wait to buy shoes the week before school starts. There may be less choice, but if you buy them now there is a chance she'll outgrow them over the summer.
HTH