The term “across” is absolutely endemic at the BBC and because so many people in UK media pass through the BBC it also crops up across the sector generally. Although I was initially scornful of it as a term I have long since caved to the inevitable and use it as well.
Being “across” seems to have originated in the fact that the BBC have multiple media streams and when a journalist talks about being “across” things they might well mean that they are producing pieces on a topic across multiple media, say television and radio. It might also mean that they are tracking a breaking story and are watching other media outlets for what they are saying about a story.
Outside this context though the word more or less means “understanding”. So when someone from the BBC is “across” something it means they understand it, sometimes if they are “across” it enough they can also make decisions about it. When someone isn’t “across” something then they do not feel they understand it or they are unprepared to answer questions about it.
Ironically this meaning then seems to seep back into broadcast journalism and I have heard journalists on air saying that they are “across” developments such as the formation of the coalition.
At this point “across” feels kind of ubiquitous apart for people who have been raised in single stream media so I think it’s worth you being “across” it too.