Having a dot com hosted in the UK will certainly help it to get listed in UK search, but there's no sure way to predict where that listing will be.
If you compare the current 'uk search' with the current 'web search', see where competitor sites already hosted in the uk fall, and that will give you a clue. But you won't know for sure until it happens.
Whether it's a good idea depends on your site, your niche, and your target audience - plus your opinion on what proportion of the UK market uses 'uk search' rather than 'web search' - depending on niche, but for most cases, probably not many.
I have a bunch of dot coms on US servers, and I'm happy with the US/UK balance; on sites I want to direct to the UK, I use .co.uk, and keep the (cheaper and more reliable) US hosting.
Your mileage may vary; there's plenty of variables to consider!
[edited by: encyclo at 12:58 am (utc) on June 25, 2007]
[edit reason] spliced from duplicate thread [/edit]