Depends what u want from it.

D50 is the cheapest on the market (at least, over here it is) and is a reasonably good camera.
Canon 350D (or wait a little and get the new 400D) is the best quality in that price range.
Nikon D70s is not as good as the 350D and more expensive.

Don't look at the sony alpha 100, it's shite and way overpriced IMO. It's just a rebranded (old model) Konica Minolta camera.

Also, if you want to get more serious later on, Canon has better lenses than Nikon.

I don't know about camera prices in the UK, but I can highly recommend the 350D.