With the introduction of broadband internet in the UK I've found it easier to spot sites hosted in the USA rather than the UK - due to the latency.
For example, when I ping our servers in London I get <20ms pings. When I ping our East coast USA servers I get ~110ms.
In my eyes and opinion, I find USA hosted websites slower, especially present when your clicking around an ecommerce store - lots of images/requests on a page. Download speeds are also limited over higher latency routes.
There's also the SEO issue which many people here will host in the UK for..
If you have a .com and want to be included in the UK searches then you need to be hosted in the UK. No one knows if google puts any weight on the fact that if you have a .uk and its hosted in the USA for example, it's still included in the UK searches - but no one knows if its affected by its location over a UK hosted site.
I will always point UK users to host in the UK and USA users to host in the US., it really depends on your target market and of course whats best for your interests/business.
Craig