ralhar - let us try this with a distinction - what you do depends on whether you want to buy a domain cheaply and sell it to someone for a profit, or.use the domain for a profitable site, and take income from that.
In the domain marketplace, let oldwidgets,com be worth $4k as a domain alone. Then, the asking price for oldwidgets,co,uk will probably be about $800, and the asking price for oldwidgets,org,uk will probably be about $80.
Now, there's a certain rationale behind these ballpark figures, the ratio between the com and the co,uk reflects the likely proportion of market share, based on the countries' populations. The valuation for the .org.uk reflects an old market prejudice against using the extension for making money, when it's use was questioned - that's been sorted out for a while now, no real problem, but domain dealers do it on their fingers ... those prices also bear some notional connection to projected annual earnings, but get they guy to explain his price and he will probably say he looked at other sites with similar traffic.
With a working site, the ways of valuation are different. Depends on who is in the niche, whether they have a site up, how much it makes and how many other sites there are in the niche, and much more. Let us assume that oldwidgets,co,uk has a parking page up saying FOR SALE - $750 and the com is also disused and parked.. There is a working site on oldwidgets,net which appeals to the North Americans - nothing about it suggests that it threatens your entry into the niche.
In this situation, you can probably take about 80% of the UK old widget market using the org.uk. If you're good at SEO, you might get to keep it ...three months into the project, the site gets 400 visits a month, converting at about 2.5% to affiliate sales (6% of an average $300 sale), so it is making $180 per month and you could proabably sell the domain + site at this stage for a projected 12 - 14 months earnings. Keep working at it, and the value of the site will increase from other factors.
At this point, the person who has oldwidgets ,co,uk will probably write and say he has noted your interesting site, and would you like to buy the co,uk for $1000 ...
for example : UK tend to use trainers while US prefer using pumps for womens high heel shoes. If your target is UK market prefer not to use the US english spellings. That will save You couple of $/£.
