When you list on eBay.ca or eBay.com your listings will have equal placement on.ca and.com, subject to Best Match criteria.
Outside of North America, your listings will only show under "International sellers" at the bottom of the page.
The only exception: If you pay the "International Visibility Fee", your listings will be included in the basic search in the UK.
If the UK is perceived as a viable market for your product(s), buy this option when listing. If it's a very weak target market, take a pass on this option.
For us, The UK now houses our second biggest customer base (we virtually dominate in UK searches and the 10 cents or so we spend additionally on listings are the biggest investment we could make when listing).
However, you need to analyze your potential. What would I do? Go for it - full tilt - for a month (buy the International Visibility, that is) and decide after that time, whether to continue or not.
Downside? Just a few bucks.
Upside? Potentially incredible.
As a certified Kraut, one of our country's mottos is "Wer nicht wagt der nicht gewinnt", loosely translating into "Whoever does not take a risk, will not win".
Good luck:-x
Xena