You can get relatively inexpensive uniform components at Woolworths or Primark or even Wilkinsons. More upscale (read "expensive") - Marks, Debenhams, Adams/some larger Sainsburys, Allders. Some schools allow "plain" jumpers/sweatshirts - other schools you need to buy from them with their badge on them.
As for school supplies, I was in shock because I didn't have a huge list like I was used to in the States! This year (Year 6) my youngest says he needs a zippered pencil case, but that's the only requirement so far this year.