DD is currently in reception. Once we were on the school system, we were invited to an after-school cake sale / getting to know you type thing, where there was second hand uniform to buy. Picked up a brand new summer dress for 50p. And she did wear it in September, as did others, so no issues there.
There were also trial days and induction evenings, where the cardis and jumpers with logos were available for sale.
I though M&S would be best quality, and bought 2 skirts from there, but was not too pleased with how they washed, also found they looked scruffy, as DD's tops would always pull out. Bought 2 pinafores from Sainsbury, and have been very please with these. Warmer for winter, look much smarter, and stop her white polo shirts getting quite so grubby.
Polo-shirts, agree with PP to buy lots - most supermarkets do a pack of 5 - ideal.
Also, don't worry too much - it's all readily available from all the supermarkets, so you won't be stuck.
One more thing, though - school shoes !!!!! You may wish to avoid the end of holiday rush and but these early, but don't buy too early, or she will grow out of them before the first half-term break . And they really ARE expensive!!