That’s the only dependency you need. languagetool-core is needed, but language-en will get that automatically. Is the text you posted really the complete error message? What IDE are you using, or is this on the command line?
This sounds like Eclipse doesn’t realize you’ve updated your pom.xml or so. You could try Languages.getLanguageForShortCode("en-GB") instead of new BritishEnglish(). If that doesn’t help, I think this is more of an Eclipse/Maven issue than a LT issue.