300 is really easily done.
Logo jumpers x3 would he 42 quid alone here
Add on 20 ( and often more like 30) for a coat. 50 quid for a pair of shoes. And 50 quid for 2 pairs of trainers and you are almost there...
And that's not including skirts/trousers , bags, stationary, water bottles, replacement pe kits etc
I dont go mad. But I also have a child in secondary who needs blue shirts and that's gonna be 30 quid for 6. ( 2x3 pack)
And aot of it has to come from.an.online supplier so that's a fiver in postage and a further 5 to send everything that dont fit back 
I am.envious of those who can can get everything including jumpers from.asda. that would be the dream.
Book bags at a tenner each are a rip off. Dds back pack has already paid for itself as it will last a second year where the book bags never did.
And as for getting away with supermarket shoes. Dd1 has nearly three miles to walk daily to school and dd2 would mash a pair of mary Jane's in a day. They just wouldnt cut it.
I spend over 200 quid on shoes alone usually. 2 sets of school shoes and 4 pairs of trainers. One for school one for Home.