I am current using cctld example (keyword.uk), the keyword is my company name, short and sweet , however keyword.com is quite strong with pr 4.
My business is locally at uk, but i now want to promote it worldwide, i am thinking of purchasing keywordunitedkindom.com , and redirect .uk to .com or should i be satisfied with .uk and promote it?
This applied to other cctld like .ca etc
PR (PageRank) does not apply to a domain, but to a web page. A typical site will have different PR on different pages.
If you buy an existing domain and change the contents of the site, then the PR will be recalculated for every page, including the home page. If your main reason for wanting the domain is to increase PR, then forget it - that’s not the way PR works.
If you want the domain to promote that your site is now international instead of UK only then that is a legitimate reason for getting the domain.
Hmmm, having “unitedkingdom” in the domain seems a backward step if you are trying to look international. It makes for a longer domain, and does the same thing as .uk in terms of signalling where you are located. All the same, a business has to be located somewhere, and I don’t think it matters much these days where it is. Heck, many trendy sites these days are theoretically located in Tuvalu (.tv) or the Indian Ocean (.io) 
, but does anyone care about that? Not at all.
I’d stick with what you have, unless perhaps you actually can get keyword.com.
Interesting choice of places given that Tuvala is apparently sinking and will soon be under the ocean.
The one thing that does make a difference with country specific domain names is that the country owns the top level domain and can do what they like with it. So if Tuvala decides to run a “stop us from sinking” campaign using all possible sub-domains then anyone currently using that TLD would be told they can’t rent .tv domains any more.
Yes, that’s one thing that puts me off using domains like that. (They always give me that sinking feeling. 
) Interestingly, I’ve heard several interviews with people who have high profile sites on .io and .tv domains, and they often say that they can’t even remember what countries those TLDs relate to or where they are. Maybe they are setting themselves up for a very painful geography lesson in the future. 