You'll maybe manage to buy the block for your 5870 for that kinda money mate!!!
Seriously though, £100 isn't going to get you much at all, and I personally don't think it's a good idea buying cheap WC components, because you will soon find that they aren't giving you the performance you were expecting, and you will end up wanting to buy something better, so it's always better to buy decent stuff the first time round, as you will save money in the long run.
You buy cheap you buy twice as they say.
It's always a good idea to keep an eye on the MM, as you can get some great deals for WC parts on there, that's where I got pretty much everything that's in my loop from, the only thing I bought new was my Heatkiller CPU block.
I think it I were to have bought everything that's in my loop new, it would have came up to around £350, but I only paid around £250 or there abouts, so I managed to save quite a bit, and everything that I purchased 2nd hand was in pristine condition to.
As I said though, your post doesn't really tell us anything, in regards to what you're planning on cooling, also you don't state which case you are using neither which doesn't help much, then there is the £100 budget....
The only thing you could really get for that cash is the Corsair H50, however, they are just the same as a high end air cooler, you won't see the types of temps you get with a custom WC loop with the Corsair H50.