Not sure it's never illegal to empty the account. If a couple only had one account, held jointly, and one paid in for a specific purpose; put £20k in to buy herself a diamond ring,say, when there was only £1 in; and the other, knowing that, emptied the account, I've a strong feeling that would be a theft! Where money is held by someone for a specific purpose, whether or not jointly, and they deliberately apply it to their own use, that's dishonest; you wouldn't say that the treasurer who runs off with the Christmas club money wasn't stealing it would you, regardless of whether he himself had paid into the club?
i wonder whether the Bank knew that you were separated and froze the account for the very sensible reason that they feared misuse of it or arguments about it.