If you're hosted in the States (for example) and are a .com site (or other country neutral domain) then simply having a .uk. sub-domain will not get you into Google.co.uk's Pages from the UK search.
Even if you put content on your UK sub-domain you'll still not qualify as a British site.
Full domain or hosting location are still the best way to ensure Google gets your country targeting right (it's one of the areas where, I think, there's a lot more work to be done).
I should point out that you shouldn't have to purchase sub-domains. You have the domain and you can make as many sub-domains as you like. Many web hosts, though, charge you for the admin costs of making the changes.
Yahoo does not like to see too many sub-domains being used. Make sure your site is large enough to begin to use sub-domains if you want to play safe.
I wouldn't keyword stuff in sub-domains either, though a keyword rich sub-domain does help.