Somebody might already have mentioned this, but try to buy the uniform well in advance, e.g. June/July because in August things start to run out especially socks, tights, etc.
At most school they start straight away with the winter uniform, eg not the blue gingham dress (which is for the Summer term, eg. after Easter).
I would not use Sharpies for labelling (I did once and it ruined a number of shirts as it started flowing and making a big blob (probably depends on the type of material as well).
If you want to order name labels then this is a good time (not too busy so the waits are not too long). Most labels you no longer have to sew in but you can just iron them in and in my experience they last very well during lots of washing - very quick and easy (including for socks).
I second that it is good for the starters to practise dressing/undressing/changing in PE clothes (including getting clothes from inside out to the right way), also this is one of the early learning goals, so it gives them a good start straight away. Also pracise putting socks/tights/shoes/coats on and maybe don't put tights on on PE days as the quickest children often get a sticker (which makes it pretty impossible if they have tights).
I would get a nice PE bag (not school one, to hard to find the right PE bag) and definitely put a key ring on the book bag, also second not to get school coats/hats etc, much too hard to find their own one.
I would get two water bottles (possibly three, to have a spare), one in the lunch bag and one in the class room. I would not give yogurts/fruit juice etc as they will make a mess in the lunch bag as they have to take empty packaging back home (so parents can check that they eat properly). Just sandwiches, bit of fruit, bottle with water and maybe a little treat (small biscuit, small chocolate), will make it much easier. Also, check how much they really eat during next few weeks, probably much less than you think! Lots of parents pack massive lunches, they only have about 15 minutes to eat it!
For the first term I would not book any after school activities as in all likelihood they will be utterly exhausted.
Good luck to you all! Exciting times! 