Hi Worried007:
If you haven't, fairly shortly you should receive a welcome letter from the school with more information and there usually will be a meeting for new parents. Quite often in July your child will actually have a visit (usually to the school, but sometimes at home) to meet his/ her new teachers.
Visit day is an ideal opportunity to have a look around and see what children are actually wearing.
We found BHS (also has on-line ordering) offers slightly cheaper but relatively smart school wear than M&S. BHS do a lovely soft cardigan with pockets - which my girls have preferred to wear all through primary school (now Y6 and Y4).
My advice is stock up on the plain stuff in advance ensuring you have the full uniform for maybe 3-4 days and then add more once school starts and your child starts to express a preference for certain clothing. For us both girls decided they wanted pinnafores for the colder weather, rather than skirts. They also wanted the school logo fleece, rather than sweatshirt.
Packs of white polo shirts are most cost effective. Sainsbury's/ Tesco's/ etc... stock school clothes now - packs of 2 PE shorts/ socks/ white polos are usually reasonable quality and relatively inexpensive from stores. Often they stock up on what the local schools require - especially in terms of gingham dresses/ sweaters/ sweatshirts/ hair accessories/ etc...
For Year R we found polo shirts were easier for our girls because all the buttons on blouses can be very fiddly. Remember that eventually they will have to change for PE - so think about that with all purchases (zippers easier than buttons on gingham dresses, elastic waists easier than button & fly on trousers, etc...).
Our girls loved playing on monkey bars/ climbing frames at school - and we picked up on a trend of buying M&S cullotes which look just like an ordinary grey skirt but obviously provide the coverage/ freedom of shorts. My girls also opted to wear old PE shorts under their gingham dresses - again for the bars.
You usually don't need pencils/ pencil cases for class - but you will need a book bag (unless the school provides it) and a bag for PE kit (usually a pull string bag).
It's probably a good idea to sew on a patch (H & M do a range of patches in their accessories section) or use a key chain/ ribbon on handle to help distinguish their bag from other children's bags (especially if the school requirers you to use their logo bags).
In terms of logo items (i.e. sweatshirts/ fleeces/ sweaters).
My advice is buy slightly big (kids grow & fast). WAIT until last possible moment on school shoes (IME feet always grow over the summer).
Labelling everything is time consuming - I find doing a little each night in front of the tv rather than all the night before school starts is less stressful for Mum! Invest in an indellible black pen/laundry marker and if you'd prefer order named labels (stickers & sew on tapes). I find stickers really useful for labelling water bottles, plimsoles, wellies, sunscreen bottle, homework books, book bags, violin cases, etc...).
HTH