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decathlon is trademarked in relation to sporting goods. decathlons.co.uk would not infringe if you simply created a site about the athletics event and had google adsense (or similar) - this may be a better option than just parking it especially if the ads are related to sports equipment.
oranges.co.uk would likely generate ads relating to fruit so registering and parking a domain is unlikely to be a problem.
Unpark it, make a site about decathalons, a few pictures of you taking part ofcourse and a bit about how much you love it and your sorted. Its a generic word so it will be hard for them to take it off you!