Even if you can see the subtitles, you will still not be able to understand certain lingo, idioms, street talk, etc., that anyone outside England would be unfamiliar with.
Certain things are untranslatable, and no subtitles could help. For instance, when you watch Japanese films with English subtitles, you are sure to see the original meanings paraphrased, condensed, transmuted into something expressible in English, but not necessarily close in meaning to the original Japanese language.