Hi,
Most present day competitions are open to anyone regardless of what country they are in. There are a few that say, "For the residents of the U.K. only," or "For Canadian citizens only," but those are in the minority. Most contests, if they advertising on the Internet, accept entries from around the world. The only obstacle is that submissions usually have to be written in English, which may make it slightly harder if English is not your native tongue.
As to what contest to enter, that depends on the story you wrote. Don't send a sci-fi story to a competition that wants fantasy or horror submissions, and also check that the story is within the word limit of the contest. In other words read the guidelines and make sure your entry meets the requirements. For example, since you say your story is sci-fi, send it to sci-fi contests.
A couple of other things--the entry fee for a normal contest is about $10 U.S., which would probably be about 6 British pounds, or about 8.2 Euros. I wouldn't pay more than that unless you were really convinced you had a good chance of winning. For one thing, contests can get expensive. No one will cry about losing 5 - 10 dollars, but more than that can put a dent in your pocket book. For another, the more they charge, the bigger the prize amount should be. It goes without saying that where there's bigger prizes, there's also more competition. I'm a little leery of sending a story to a contest that could get 1,000 entries, simply because my odds of winning won't be as great as submitting to a contest that gets 200 - 300 submissions at most. Some contests are lucky to get 100. These are just things to consider. Try to pick the contest that sounds like it's best suited for your story.
As to places to check out good contests, I'd go to Ralan's at
http://ralan.com/contests/
and also I'd look at the Preditors and Editors page at
http://www.anotherealm.com/prededitors/pubctst.htm
Both of these sources will list legitimate contests.
Hope some of this feedback is useful. Best wishes, Terry