The Xbox 360 hasn't come out in the UK yet, but this is what I know....
There are 2 packs - the less expensive CORE pack, and the more expensive PREMIUM pack. The Premium pack contains things like a hard disk, headset, generally loads more goodies, and that's more or less sold out at the moment in the US. The Core pack is cheaper, but there's no hard disk (see next point).
As long as you have the PREMIUM PACK (which was the hard disk) you can play around 200 Xbox games on the Xbox 360. Apparently, data is downloaded from Xbox Live onto the hard disk to make the game playable, and Microsoft intend to update this list and make more Xbox games playable as time goes on.
In my advice, wait a couple of months. You'll have a better idea of how well the 360 is doing, plus, in every console launch, the first batch of units can be a tad unrealiable at times.