I don't think the BBC has any valid case here, either. But a corporation doesn't need to have a good legal case in order to make trouble. All they need is money to pay their lawyers and the will to pursue legal action. They can make enough trouble to shut down a production simply because nobody wants the headache of dealing with that shit.
So this is less about whether the BBC has a valid case than whether they have the resources to make as much trouble as Sony could have. That's what I find doubtful.
However, CBS has just renewed a ton of shows, and they have a fair number of pilots in contention, which means any single pilot is going to have a big uphill battle to get a series order. If the BBC made any trouble at all, that could be enough to get CBS to pass over Elementary
in favor of something that won't be any legal bother at all.