How do I distinguish a red kite from a buzzard?

I'm not really a bird watcher or anything although I do know a little bit about common British birds.I'm just wondering if I actually saw a red kite because I read that they are very uncommon.the only thing I saw was a large brown bird that was white under the wings.I never really got to see the shape aswell because I was in a woods.I also live in north Wales countryside if that could help find the answer!!