since amazon has a .co.uk address i'd go for .co.uk

Good idea.
But, whatever it is that you use, it would be nice if you first build the brand. how the market there?.

Maybe it's just from my simple opinion.

Regards,

bigbabol

When you're doing your keyword research, make sure that you select to view results from UK searches specifically to see if the niche is worth entering into. Although the market is bigger for US items, some niches may be easier to break when you're targeting UK customers. It all depends on what you find during your research, so it could definitely be worth buying a .co.uk domain if it's available.

Also, you may know this already, but if you're signed up to the UK associates then only site visitors from the UK can buy the products you link to anyway. But you can also sign up to a separate US associates account if you want to link to the US store.