[susan53]

RP is a form of pronunciation rather than anything to do with grammar or vocab, and actually is not the standard British pronunciation any more. But if what you mean is that you need to teach British rather than American English, I wouldn't worry. There are very few diffences which are incomprehensible, and a lot of American influence on British English, especially from American TV programmes. British people would know exactly what you meant if you said gas station.

I'm interested though in why you have to teach British English? I'd have thought that most learners now need a more neutral international version of the language, as in most cases it's impossible to predict who they'll later need the language to speak to - it's as likely to be the Chinese as it is Americans or Brits. Being British I inevitably have to teach British English - but where I know there are variations always point them out. So my learners know about gas stations as well as service stations for instance, and know the difference between an American subway and a British one. As they're as likely to travel to New York as to London they need both.
What do other people do?