My boys wear generic white polo shirts which I buy from Asda or Aldi / Lidl. If they are particularly stained, I'll throw them or use them as household cloths. I don't mind buying them as they are cheap.
They do have School jumpers which have to have a logo on. They are £10 each. A couple have stains (small pen marks), which aren't that noticable so I use those as a back up (as in, "crap, I haven't washed the school uniforms in time!") They wash really well so they don't stain easily (from food, paint etc)