Does the phrase "I feel you" sound too slangy and somewhat horrible to a British person? Is it ok to use it as a synonym of "I understand what you feel/say" in an informal, casual conversation?
No, it doesn't sound too "slangy". However, unless you happen to be making a fun remark that befits a reference to pop-culture (American pop-culture to be specific), or you happen to be a proponent of said culture, or you're so inundated and surrounded by this "culture" that in some situations you deem it easier to just get along rather than fight the current, then it may convey an identity crisis! Asking for other people's opinions regarding this specific issue might further suggest the presence of an identity crisis. Having said that, identity crises are known to occur and recur in humans roughly between the ages of 0 and 110, so it's nothing to brag about! Cheers.