The choice of TLD will not have any influence on rankings i.e. a .com will rank better than a .net etc is not true... Its only because of the massive number of .com sites out there that such perceptions, myths arise... Google and other search engines rank your website on factors such as the type and quality of backlinks (i.e. off-page SEO) as well as various on-page factors.... the type of TLD is not a factor...
As RDO mentions above - the domain name can help with ranking for a particular keyword.... With that said - if there is already a website with the .com name and you are looking to get hold of the same name with another TLD - then be careful that you are not violating copyright, trademark's etc.... On my part I would avoid it completely as many people will inevitably mistake your site for the .com version - so chances are you'll be losing some traffic as well....
If your primary market is UK focused - then co.uk is a good option - however if it is not then dont go for it... Most people that see a site which has country level domain i.e. co.uk, nl, de etc will assume it is not applicable if they are viewing it from another country.... Do note that just because a site has a country level TLD it does not mean it is restricted to that country... i.e. co.uk sites can rank on google.com with out any issues as with google.co.uk... its just that national TLDs tend to be skewed towards businesses which are only targeting a market which is either at local or national level - hence another perception that such TLDs help with local rankings which is not the case (its whats on the website and what links point to it which matters)....
