I wished I had bought uniform earlier - loads of places had sold out of the right size in the week before term starts.
However, secondary schools are VERY particular about uniform rules, so wait until you get the full kit seet at the induction evening. They may even specify what the bag must be like - black, no logos, at DS's school.
I didn't realise Ds needed a pencil case with calculator ('scientific' model), geometry set, pens, pencils, crayons etc.
Coats also - they may specify type and colour (black / navy, no logo!)
It all seems very scary and a lot to take in, but it all falls into place, bit by bit.
Find out the arrangements for lunch - we had to put an initial payment down on the cashless card at the Induction evening, but every school will have it's system.
Read the planner every night! The Planner is a diary they have with them all day every day and they write homework in it and any other notes and things to remember. Then they forget to tell you anyway - so check the planner!
Buy 2 ties. They get lost. Somehow. And there is no leeway in secondary for lost ties.
Make sure they know how to tie it after PE.
Chippy are you roughly in the same area of S London as me? I'm not sure why I think that.