I don't have any actual experience, but I don't think search engines would treat them any differently (probably the same as having a .co.uk)....the problem is people remembering the actual extension.
This could be a problem for you as I assume the .com and .co.uk equivalents have already been taken.
Actually, looking at those results there's some good sites. If your website is actually a UK organisation, like those listed, then I'd probably go for it.