"Accidentally" isn't certain here. If I was part of something that was wrong and I wanted it to be known, I would very well "accidentally" leak it too.
Except I don't see how that applies in this case. Stay or leave -- it's not the bank's call. But if politicians are putting leaving the EU on the table, even as an empty gesture, then naturally the bank has to start thinking about contingency plans. That's just common sense, even if you think the very idea of leaving the EU is mad.
It's also common sense to keep that on the DL to prevent misguided overreaction to what is after all still a hypothetical scenario. The Bank of England a central bank and so people must be constantly scrutinizing it hoping to glean inside information on future monetary policy. That's to say nothing of having to deal with the conspiracy theory nutters.