It's a good question, I generally use .co.uk for most things - finding information. shopping etc I find that a .com search throws out WAY to many results
Another thing to consider - and you will probably already know this but some people may not...
The location of your hosting can have a great effect on your country specific search rankings... I will try and explain but it gets a little complicated!
If you own a country specific domain name extension - .co.uk for example then Google knows that your website is UK based, or at least targeted at a UK audience so it lists it in UK ONLY searches from Google.co.uk
If on the other hand you own a GENERIC domain name extension - .com or .net for example then Google has no way of knowing your target audience so it uses something called geo-ip location which basically means it looks up the IP address of the server that your website is hosted on and uses the servers location to determine the "target" country. so if you have a .com on a USA hosting account then you are in trouble in regards to UK only searches
BUT...
Having said all that there ARE a few .com's listed in UK only searches that are located OUTSIDE the UK - I'm not 100% sure how or why but I am guessing it has something to do with the % of inbound UK links?????
Also there is evidence to suggest that if you take 2 identical website's (content, PR, inbound / outbound links etc) and host them in different physical locations then the one on the UK server will outrank the non UK hosted one in a UK only search.