That's a tough call! I'm more inclined to recommend the Nikon over the Sony, however as doubledee has stated there's not much difference picture quality wise between the two. Did you know that Sony makes the image recording sensors for the Nikon cameras, and all others except for Canon for that matter.
Sony did buy out Minolta and the A100 is just an incarnation of their legendary maxxum digital series. I got into the DSLR game shortly after the takeover and have come into contact with many die-hard Minolta users. It is true that Sony's new DSLR lenses are practically extortionist; but Nikon's can be as well.
I own a Pentax K10D and I have to say that I'm completely thrilled with it. I don't know what kind of deal you can get on the Nikon over in the UK but here in the USA the Pentax is usually about $100 less than the D80 and right now it's about $200 more than the Alpha A100.
The difference between the K10D and the D80 is based around 2 things. First Pentax accessories are much less expensive than Canon and Nikon accesories. Lenses that cost $1,200-$1,400 dollars from Nikon and Canon can be had from Pentax for $600-$800, these lenses have the same build quality and generally score as good or better than the Nikon and Canon counterparts. The second difference between the K10D and the D80 is shared with the A100, this being in camera image stabilization. In case you don't understand what that is I'll give a short synopsis.
Cameras such as the Pentax K10D K100D and K100D super, Alpha A100, and Olympus' new models have what is known as in camera image stabilization. This feature functions by physically moving the image recording sensor to compensate for the natural unsteadiness of the human hand. The camera is equipped with gyros that sense the direction and intensity of the "shake" and send the information to a computer that operates a series of magnets that push or pull the sensor in the direction it needs to go in order to have the image come out crisp and blur free. This system was originally mocked for being less efficient than the systems offered by makers such as Nikon and Canon, but it has made huge strides and appears to be catching on with consumers. Nikon is rumored to be listening too, I've read sneak peek rumors that state that the Nikon updates to the D80 and D200 will have in camera image stabilization. Pentax has come into their own in the DSLR world on the back of this technology. Pros of this system are mainly in that it works with every single lens you choose to attach to the camera.
The Pentax and Olympus in camera shake reduction systems are the better of the two, this being simply because they are able to compensate for shake in not only the up-down and side-side directions, but also in the corner-corner directions and the Pentax can even compensate for a swirling pattern which is teh most common shake pattern with long focal length lenses.
Canon and Nikon have been touting an in lens image stabilization system, this meaning that the elements in the lens move to compensate for camera shake and keep the image centered on the image sensor. It works well, but is is very expensive. For image stabilized lenses you can expect to pay an extra one or two hundred dollars. Also it is beginning to come out on the review sites that image stabilized lenses are much more prone to mechanical difficulties than non-image stabilized lenses. It would be terrible to pay a lot extra for a lens that is less durable.
If I were to choose between the D80 and the Alpha A100, I'd pick the D80 simply because I've handled them both and the build quality of the D80 appears to be better, also the viewfinder in the D80 is much better. Image quality can be tweaked in-camera to come out the same in any DSLR out there. If I were to pick out of the crop of DSLR's out there and not just these 2 I'd be getting the Pentax K10D again.
SLC