I would go for one with USB, since Serial is going the way of the dino. But It seems to be a good price (I wouldn’t go higher than $70 USD, which is about 40.53 pounds)
The middle grade one looks to be the best (the Wacom Graphire 4)
The cheap one just lets you write to the screen, but the middle one lets you actually have sensativity in the pen. So the harder you press the bigger the stroke, things like that. And I also think that it has the angle thing, so with the proper program it will splatter out lke a real airbrush.
Anyways, thats probably your best bet, I don’t think the one above it (Intuous 3) has anything mre than the one I recommend then some little shortcut keys.