The issue is whether you're targeting US or UK, or otherwise.
A UK IP will do you better good in google uk. What it doesn't do, which is what tedster was saying, is affect any ranking from the point of view of on-the-page- optimisation. With other sites positioned differently in the UK serps it'll position the site differently according a to a number of factors, not least being the competitiveness of the target keywords. It might be much more competitive in the Uk, but less so in the US, therefore positioning will be less good in the uk.
Clear as mud, eh!
Either way, the IP assessment is not always that accurate.